why is the 30k models more detailed?

they are from forge world....so resin cast!

WALL OF TEXT WARNING:
Only reason the xenos aren't played in 30k games is because their codices are inadequate to cover what was occurring at that time. The Greenskins had sector spanning empires that had been/were in the process of being destroyed and purged by the Imperials. The Space Elves had only recently (to them) lost their empire and been forced to scatter and adapt. If the terrible overlords of GW would make 30k era codices for the Space Elves and Greenskins then that would be awesome.