You could have read a couple negative reviews to answer your own question lol...
Briefly, the mains reasons were:

>Hackers.
You could find at least 2 of them in every single game a few months ago when the anti cheat was pretty fragile. Presently there are fortunately way less hackers than before.

>No regular and or big updates.
Uberstrike must be Cmune's last prioprity as they haven't updated the game for years, for real. No wonder why they simply handed the game to a community development team.

>Sync, laggy servers. I guess there's no need to elaborate.

>''f*cking P2W''.
Now thats a recurring false statement.
Newcomers who make bad use of their startup coins (ex: rent guns only) will inevitably get their ass kicked and start blaming the ''P2W system'', totally illusory. It shouldn't take much time to buy any casual item permanent with a bit of effort and perseverance, at least, for players who played a FPS game before.

iRevolutionz said it all and I have the about same opinion. I still will write down my answer to confirm.

tl;dr: Look at the last paragraph.

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In fact UberStrike is 5 years old, if you count in the versions before the renaming of the game (former name: Paradise Paintball) it is even 7 years old. The first and greatest first person shooter on facebook. The glory died about 2013, I guess.

Some time before the game made it to Steam, (I estimate 1 year+), the devs (CMUNE) declared UberStrike as their lowest priority. They put UberStrike in the hands of some community members, called CDT (community development team). CMUNE called that move "maintenance mode". In fact CMUNE didn't work at all on UberStrike and the CDT consisted/consists of people working voluntarily in their free time, not getting any pay and without having all the tools.

To me the designation "maintenance mode" was a joke, at its best. As there were no major updates to get rid of bugs or exploits that hackers used. I don't blame the CDT for this, but it has to be said that few members of them used it as stepping stone in their career. Facing these facts it is no wonder that the game became one of the greatest hacker-/cheaterfests around.

And then CMUNE finally released the game on Steam (17th June 2015), it has been greenlit already long time ago, even before the "maintenance mode" if I remember correctly. To me it was a big mistake that they didn't push through the update before they released UberStrike on Steam. Due to the great cheater issue many legit players turned their back to this game and got mad at it or they started to cheat themselves. I guess that's what most bad reviews are based upon.

Finally, some days later (26th June 2015), an update has been pushed through that got rid from almost every hacker. A good move, but hackers wouldn't be hackers if they don't find new backdoors in the code and they are slowly reconquering the game. Basically every review yelling about hackers that have been posted between 26th June and let's say around the end of August simply are false. I guess mad players or trolls. For the very current situation the devs and CDT need to come up with a new update soon(TM).

Enough flashback / history.

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Some words about sync/lag issues. You simply can't hinder that there is any latency on the internet. People complaining about it either never realised that the internet isn't instant or, in most other cases, it was a client-side issue. I personally never really had sync / lag issues.

But there still is an existing issue about servers. Many years the game had multiple servers in Europe, South America, North America, Australia and Asia. Maybe 15 in total. Already some months before UberStrike has been released on Steam the game lost its servers on some continents. In fact there are only 3 in total left. One Asian, one European and one North American server. Now there is a good chunk of players that need to join a server which is located on another continent. It is purely normal that their latency is higher and that their lag is greater. But it is unbearable that they don't get their own server on their continent because it would get rid of a big part of synch / lag issues. (Note that Africa never had a server at all, iirc).

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Pay to win is history since the game made it to Steam. There are 2 ingame currencies, the freely obtainable points (a.k.a. coins) and the credits you need to buy with real money. The two reasons why the game can't be considered as P2W anymore are:

• Every item (except name change) is now obtainable with coins.
  
• Prices of coins have been cut to about a tenth. That means that you can buy 2 or 3 of the most powerful weapons with the coins you obtain at the start.

Needless to say that every review complaining about P2W is, frankly said, 100% false. One advice though: think good about what to buy before you spend those coins. I made a (almost complete) guide on weapon stats that might help you. I may post a link if you want to. (Without being asked I won't, because I don't want to do shameless advertising. I know, I'm no good at PR).

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Now I want to add something that iRevolutionz didn't mention. There are quite a bit players that are disappointed about game mechanics that have been removed and I feel them. If they complain about things that have been removed, something like the team elimination mode, the shop's weapon testing area, weapon stats in numbers, weapon pickups or several maps, that is totally legit. Heck even the complaints about the removal of quickswitching would be a good reason to give a negative review, (which I personally consider as a bug that persisted many years. This is another story, please no flamewar on that, I feel you and I stated it is okay to complain about it).

Inb4 Team Elimination is planned to come back.

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Yes, the game has some annoying bugs, that still need to be fixed. Some of them have been removed since the release on Steam.

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tl;dr:

It seems that a good portion of bad reviews (I guess at least one third of them) are unjustified. That would make at least 58% positives and 42% negatives. Quite a bit of players are mad or disappointed about the game already before it made it to Steam (17th June 2015) and didn't try it out on Steam, especially after the major patch (26th June 2015) which got rid of some bugs and many cheaters. Note that the hackers find new ways and the scum is returning as we speak. The game is nowadays by far not as fun anymore as it was 2011 but worth a shot. I hope the devs will continue to work on it and won't let it down for over a year as they did before UberStrike's release on Steam. I still like the game but if there will be no further major update till the end of the year, I would consider the game as discontinued or dead.

I hope the wall of text helped you.
Cheers