Easy question are answered quite quickly, there's a small competition on them, to get easy reputation points... But these will usually get zero score.
Better questions, which are usually harder, get higher scores, which gives motivation to ask better questions in the 1st place.
Here's the thing, if there are fewer experts on certain topics, There's less scoring on the questions and less solutions on questions that are a little harder.
The only solution that I'm familiar with, is the bounties, but isn't there any other mechanism, to encourage more experts to arrive to the site in the 1st place?
This is true less for StackOverflow - more depending on the specific field But even more so on sites with less users and less traffic...