Actually those who have seen it have said it is pretty decent.
1) not everyone who has seen it has said this. anagram said it sucked. the audience score on rottentomatoes is 59% at the moment. 2) "pretty decent" is not good enough. lots of other movies that play in theaters are "pretty decent" too. personally i expect more from a star wars movie.
To be fair, Anagram usually says everything sucks, so that's not a huge surprise. And audience score typically isn't a good indicator of how successful something will be, since TLJ is at like, a 47%, there.
Perhaps that 47% audience rating for TLJ is why this one is doing poorly. They didn't like the last one so they aren't bothering to see this one.
Anyone saying TLJ has nothing to do with it is absolutely naive. ---