But that's the thing: they never introduced new mysteries and answered pretty much every question left over from the first. A good story works on its own and the original Exorcist was meant to be self-contained (until Blatty himself decided to do a follow up that, really, isn't actually a sequel. It is more a spin off).
Another aspect is that sometimes sequels (especially if they are shitty ones) detract from the original work and diminish its standing. There are plenty of questions left over from The Big Lebowski. Why haven't we ever got a sequel? Because it doesn't need one (The Jesus Rolls was a spin off and did nothing to add to it).
There is such a thing as flogging a dead horse and this is it. Go watch all the Exorcist stuff and then come back and tell me what they added. Exorcist 2 was done on the cheap and they straight up just wanted to rehash the original. It was such a farce that Blatty himself was the first person in the cinema that laughed when they screened it.
This is a problem I have with Hollywood as a whole. Some stories are tailor made to have sequels and become a series (which is fine). But then they get hold of one that was never written or intended to be a franchise and really drive it into the ground.
Citizen Kane left a few questions, does that mean it should get a sequel? What about The Great Gatsby?
Some stories end and, if there are questions left over, the author prefers to leave it up to the reader's imagination to what the answer may be.
There is a reason so many here are rolling their eyes at this and it is because each attempt to add to it (3 sort of dodges it) have been poor. It'll take a miracle to get people to be enthusiastic about this and, honestly, I don't see it happening.