Geoff Goodman made a post on the official forums discussing the suggested changes to Wraith Form:
“Its not out of the question, but there is a benefit to having these sorts of ‘all in’ feeling abilities in the game at large. Other similar abilities are Roadhog’s heal and Reinhardt’s charge.
We could make these types of abilities cancelable, but part of what makes abilities interesting in Overwatch is how they often come with certain drawbacks, which enemies can then capitalize on.
Pharah’s ult, for example, is one of the most damaging attacks in the game, but comes with the considerable drawback that she is somewhat of a sitting duck while using it. Using her ult well, then, becomes a matter of balancing dealing as much damage as possible to as many players as possible, with trying to make sure you aren’t killed quickly after hitting the button. You’ll see pro players hovering above a door way just before enemies stream through it. Unleashing her ultimate from that position allows it to target a large cluster of players while still giving herself decent cover.
For Reaper’s case specifically, his Wraithform ability is very strong, but at least you know he can’t attack you while he himself is unable to be attacked. Currently, you can try to time a shot to land just as it ends, if you’re practiced enough. If he were able to cancel it freely there would probably have to be a decent delay before he were able to fire again, to give his enemies enough warning. Otherwise on your screen he’d shoot you before you even saw the effect end (at least until someone invents 0 latency internet).
Again it isn’t completely off the table for him but i’m trying to give you guys an idea as to why it is the way it is to begin with, and that removing these ‘costs’ that abilities have isn’t always good for the game as a whole.”