There's not a specific reason for it, but the reason Any% No Telejumping is run is because it's the "classic" VVVVVV speedrun. 100% No Telejumping would not be significantly harder than 100% with telejumps, only longer. The reason for this is that you can't "cheese" a level like you can with Warp Zone and Tower in Any%, because you still have to get the trinkets, so the only time you'd lose from skipping Telejumps is from walking to the level.

personally, excluding stuff like the double intermission (principle) skip, telejumping makes the category more interesting in my opinion. it makes space station routing cool, easier access to tower and lab. and once you get the hang of telejumping, it isn't too hard.

Also skipping other glotches would be good