why do most gamepass pc/Windows store games not have available cheats when their steam counterparts do?
It’s because despite gamepass being a great offering,the Windows store is not very popular as a gaming platform,at least compared to Steam.From what i’ve seen in other posts Wemod uses a software to calculate how many of its users own a specific game and for which platform,and if said game reaches a threshold a trainer for it will be added to the queue.It’s possible then that some games simply haven’t met this requirement on the Windows store but have on Steam.
Exactly what Don said, its why you can vote for games to either be updated or even have cheats created for them but each game does have a required number of votes before the system flags it for examination but even then it can have its position on the list changed at any time depending on the votes as i have seen games rise and fall rapidly
Hello! 
As others have already said above, Windows Store is a very unpopular platform.
- For example, one reason they’re disliked is because they deny you ownership of your own game files for all the games you buy from them, unlike other stores.
- The Windows Store is also full of junk, like very bad mobile game ports, because Windows Store is too relaxed and has no quality control - whereas Steam, Uplay, Epic, etc do.
- Other platforms, like Steam, for example, also has a much larger community, meaning more people to play with.
And so on, I could give you examples why Windows Store is just plain awful all day.
Trainers are only made if the game is popular enough on that platform. Making, maintaining and hosting trainers costs WeMod money, if the game is unpopular then WeMod is not going to get a return for all the effort put into developing the trainer, hence it is what we, in the business world, call uneconomical.
This recent post explains how getting trainers made works and how you can improve its chances: Vampyr for Windows suggestion - #2 by Ravenfyre.