Brand recognition people. A game with a name people will recognise will sell better than a brand new IP. If you want evidence of this, look at all the original games that are brought out, get rave reviews from the critics, but crash and burn in the sales charts.
I don't know why it always surprises people when companies do this. It's the same reason games with completely unrelated stories and characters will retain a franchised name. For example, look at Final Fantasy: different characters each game, no over-arching story. Very little, in fact, to tie the games together. Yet they are still called Final Fantasy, because if they came up with a new name each time, they wouldn't sell nearly as well.