There the same locomotive(give or take a few extra working buttons/switches).Yet the ECW br class 66 get the livery editor and GWR br class 66 get the shaft.
They are two different models in the game. One has been updated and one hasn’t. There are plenty of other trains that haven’t yet been updated so you’ll just have to wait for the one you want like everyone else. If you are madly desperate to repaint the Class 66 you can buy ECW.
Because (as far as the game knows) they ARE different locos, at least different assets, and as stujoy said, one's been worked on and the other hasn't as yet. The same is true of the BR Blue 52 and the WSR 52, two different assets even though they're simulations of the same locos
