Apparently Boba Fett was too weak and so was his gang. Why should Vader not have sent his troopers to take out Luke, Han, Chewbacca, and R2D2 who later helped Luke to escape? That way he could have prevented Boba from dying, and the Star Wars series could have been ended with Imperial victory.
I don't know
and don't care what Disney canon says on the matter. Likely nothing since to the best of my knowledge no new Disney material covers ROTJ time period except actual Episode VI film/novelization; and they say nothing of the matter. Earlier script drafts detailed in "Making of..." book had some interesting ideas but nothing I found useful to answer this question.
In Zahn's work (first Thrawn trilogy), we know that the Emperor DID cover his bases - he sent Emperor's Hand, a trained Force-sensitive assassin Mara Jade to keep an eye on the Jabba's bachelor pad and make sure Luke gets executed or captured (for obvious reasons, the Emperor didn't much care about Han's fate).
Mara Jade ran into the typical Jedi problem: Jabba was a Hutt. Stubborn, annoying, and not prone to falling for Jedi Mind Tricks. So, he - for some unstated reason - refused Mara Jade entry onto the barge the day of execution.
According to Luke's own Force influenced dreams, if Mara had been there on the barge, he'd have been Sarlacc nutrients. So, in a way, Jabba saved Luke's life. He should have gotten the Force Ghost at the end of the film instead of Anakin :)
Because that would provoke the Hutt cartel. They are a vast and enourmous criminal Syndicate, interfering with their business would likely cause war