It's a combination of various factors. The games definitely play a role (PS3 has more exclusives, Xbox360 is mainly Halo and even that has been remastered on PC now).

Another factor is how the projects are maintained and managed. The maintainers of RPCS3 are more active and pull requests get feedback and merged relatively quickly.

On Xenia side, pull requests can remain open for months even though they should be merged, and this has caused multiple (potential) contributors to stop making pull requests.

Source: I'm a contributor on Xenia

Complete guess: I think part of the issue is that a lot of Xbox games that aren't exclusive to Sony or Switch are also on Windows. So maybe there is a slightly smaller incentive?

It is also interesting that the orig xbox is not further along.