Because it's a sequel. People are expecting legacy characters--Han, Leia, Luke--to come back. It gives another point of nostalgia as well for older Star Wars fans, upping the emotional impact when he passes. Also, this is the Skywalker saga, and Luke is the only Skywalker left at that point(many people don't count Leia and Ben).
I don't see the problem having Luke be the mentor. He has an arc in the Original Trilogy, and then another in TLJ. His new journey doesn't undermine the old one.
Final point: a good rule of writing is to eliminate extraneous characters whose role could be filled by a character already part of the story. I personally think Luke is an excellent choice as a mentor character, and having another character wouldn't really make sense. Rey comes to Luke to be trained as a JEDI. As far as we know, he's the only Jedi Master, let alone Jedi, left at the time of TLJ.