Aside from the problems already mentioned, there's a few reasons why the Sarlacc would be a bad disposal method. The reasons for disposal post-mortem have already been mentioned, so I'll stick to live disposal.

First off, the slow digestion of a Sarlacc means survival and escape are possible. Second, the Sarlacc is sentient. It feeds off of and learns from the things it is digesting. And if a Sue is being digested... well, a possessed Sarlacc won't exactly be easy to exorcise, and will be a greater threat to the agent than most Sues.

Plus, slow digestion could count as torture. A death of a thousand years...

Also I imagine a reason to not do it, is that in previous canon, before the Disney Reset, Boba Fett actually survived the Sarlaac, so there is some precedence for failure. Of course I just realized you probably mean disposal of the body, meaning the Sue is already dead, and for that go back to the reasons previously mentioned.

In general, it's preferred that the Sue be disposed of within the continuum she invaded in the first place, or somewhere neutral like HQ. No need to risk spreading the contamination around. Plus, we try to keep things fresh as much as possible. Reusing the same disposal method over and over would get dull, especially in a universe as large as the SW universe. There are practically endless options to choose from, so why limit yourself?

Also, yes, kindly do slow down. As iterated below, each new post knocks an older one down and finally off the front page. Please add new questions to an existing thread. I promise they'll still get noticed.

~Neshomeh

Anyway, we don't use the sarlaac more often because 1) Even with her high disposal rate, canon monsters can get fat from too many Sues, 2) We like to see variety and creativity in the disposal methods, and 3) Cross-canon executions/disposal is generally discouraged and we don't have too many Star Wars missions.