purely assumptions.
i don't know how to code or do any image reconstruction *or whatever its called* but i'm guessing in order to support a higher res you'ed have to go into each image ex. building, character, gun, bullet, box, etc. and redraw each one in a higher res then write a program that'll calculate the difference in mouse movement, speed, etc to the original res and work out bugs and all other little details.
also i'm not a 100% sure but i think xtrap goes into the res folder and rechecks to make sure that the original files are still there and not anything new that you made. you could probably bypass it by injecting the files but the amount of files needed to replace all of the res files would slow down your game start up substantially.
lastly you would need the coder to update each time the game updates. and just like the pubs, leechers would take it and not give credit making the coders frustrated and stop coding it all together and make it vip.
so considering the amount of work and the minority of people that actually want it i don't think any coder would be making this anytime soon.