First, for those who don't know, I want to point out Mod Manager's official documentation on using a Manifest.xml: https://github.com/fluffy-mods/ModManager/blob/master/ForModders.md (BTW: After customizing it for your mod, you put this in your \about\ folder.)

In regards to having Cross-Promotions appear - a "mod showcase" of the mod author's other mods - this document says:

showCrossPromotions

If true (default), Mod Manager will show a bar with previews for mods you have uploaded to the Steam Workshop that the user doesn't have installed (or active, if the user enabled that setting).

NOTE: since the crosspromotion system relies relies on the Steam Workshop to fetch mod details and previews, it can only be used with mods that have a PublishedFileId. That means they either have to be downloaded from steam, or contain a PublishedFileId.txt file containing the PublishedFileId in the About folder of your mod.
NOTE: users can toggle the cross promotion feature off regardless of the setting you choose here.

To have a mod show cross-promotions, it merely has to have a Manifest.xml with the line:
"<showCrossPromotions>true</showCrossPromotions>"

Here's an example from one of my mods:

My questions:

Aside from the presense of a Manifest.xml, having a <showCrossPromotions> tag set to "true" and "PublishedFileId.txt" file containing the PublishedFileId, what else determines whether or not a mod author's mods will show up in the cross-promotional field?

Currently, I have 16 mods for RimWorld, of which 5 were either updated for or released for [1.0]. However, in the cross-promotion field, only 1 mod shows up:
Retriever, Go Fetch [1.0].

In contrast, I see that popular mods have many more than 1 mod show up in their cross-promotion. I see anywhere from 3 to 20 mods in the cross-promotions of other mods.

So, what am I doing wrong? Why is it that I only see 1?

I seem to recall that, originally, zero mods showed up in my cross-promotion. I know for a fact that I have <showCrossPromotions> set to "true" in my recent mods. Though, I will admit that, originally, I had <showCrossPromotions> set to "false" on a couple of my mods until a couple weeks ago.

Is the reason that I see even 1 in my cross-promotion because this mod was only released recently? Does it have to be recent? Does a mod have to reach a certain popularity, view count or download level? Did it help that I released this mod as a separate release for [1.0] help (instead of updating it for [1.0] like I did for another mod)?