It also have to do with the fact that fractal is restricted by 5 men, so usually one person have to take on more than one responsibilities (stability, protection, might, alac, quick, reflect etc....). I assume you're playing in pugs where dps usually only dps and nothing else, in swapping support you'll likely be missing one of the crucial component to the comp unless you have someone else in the team take it. Since fractal is also a daily thing, people also just want to clear fast and don't have to spend time working out who's taking what. And groups that lack those important buffs and utils can take too long clear especially if your team lacks experience.
But I agree that experimenting with comps with super fun, but it's hard to find the right people