The difference between them for you is often dependant on your approach to impro. Obviously, at the core, is the simple principle of "Yes, and." But on top of this, there are differences in approach that players and groups have to using the "yes, and." And some of these approaches best suit one or the other of the forms.
One key element to explaining this is that shortform is very forgiving. Any scene you are in will only last a couple of minutes and then you can get a whole new suggestion and start a completely new scene. So any stinkers can be moved on from and soon forgotten. Even without stinkers, shortform is very tolerant of scenes that are not about anything, have no clear characters or relationships, or have characters and relationships we really don't care about. We can enjoy the reality being destroyed through gagging, commenting or simply playing with a lack of conviction for those three minutes. In short, we can happily watch an undefined bunch of funny stuff for 3 minutes, as long as after that three minutes there is something new.
If you want to stretch it out, however, and make the scenes longer, the characters return, the audience to care, then some of the very things that shortform tolerates, and usually the very things that can get the biggest laugh, will destroy all your efforts. Comedy, after all, is very often about setting up an expectation and destroying it.
People can be phenomenally strong shortform players and struggle with longform because their approach, the way they harness the "beast" (as I've heard it called), is actually detrimental to creating something that can sustain itself longer than 3 minutes.
Things that get laughs and keep shortform safe, that are detrimental to longform:
- Commenting / mugging to the audience.
- Playing like it's all a joke.
- Superficial characters who aren't affected.
- Undefined relationship.
All of which can be summarised in the single statement: "Lack of commitment."