Although I am an huge fan, I do feel Scirra could put a little more effort in keeping tabs on community projects and their progress, and by extension keep their showcase updated.
Or it could be the matter of relative difficulty exporting finished products which has been discussed often.
I find it more likely that successful long time C2 developers are busy being successful, rather than promoting the engine they work on. I imagine for the developer making the blockbuster there is little incentive for them to promote the engine they created their product on. It falls back to Scirra to promote the success stories.
On the other hand, Scirra may be making enough from licensing and the store to not even care about pulling in (and supporting) more users, and focus on C3 instead. It all comes down to economics and incentive/motivation.
Edit: PS - There are actually many more reasons for people to want to NOT show the "Powered by Construct 2" splash screen, given that they are not REQUIRED to, unlike some other big name engines like Unreal ect.