im guessing because thats overpowered and the new one is more accurate

yup. That was too OP. Reactor packs contain reactors balanced for new weapons.

its a bit annoying when most of your ships use said overpowered thing and then they don't work anymore

I feel the pain resonating from this thread. I loved my motion core generators. Super sad to see them go, they were very fun to use on creative mode. I think they'd be worth downloading as a stand alone mod. I don't want balance lol, I want POOOWWWWEEER.