Motivation for modders.

Donations are made to a special account belonging to the portal administration.

The main bottleneck is that no one will want to do this, I believe.
It would be better to use something like Patreon (or use Patreon itself). Whoever implements it gets everything. If they don't implement it or the implementation is not of high quality, they don't get anything.

However, there are many requests, but only 1% of them are fulfilled. Why so few?

This is actually normal (I doubt it's 1%, I think it's 5-10 percent). Many requests are physically impossible to implement, and others have a poor ratio of "development effort/audience reach."
Thank you for the idea.