is there any specific reason for why almost every poison type pokemon can learn thief but umbreon can't?

Hey @Skythekid8

 

If you got any Pokemon PRO related questions in the future.

Feel free to post them in General Support subforum next time.

This is both for faster response time and for keeping questions in the right subforum.

 

Now to answer your question.

While its true that Umbreon can learn the move Thief from TM/TR in VIII. But in any other generation its not supported as a TM.

I'm not entirely sure which Pokemon version PRO wishes to follow, but it could be that this is due to it being farily newly added as a TM/TR in VIII.

 

If you want, you could suggest to add support for it in the Suggestions subforum.
But be sure to read this post before posting a suggestion.

 

Hope this helps answering your question!