Because he's nearing the end of his career at the top, and has maybe a couple of years left. Does he want to spend them at a team in decline, like Barcelona, or does he want to play for one of the best teams in the world. Barcelona are unfortunately not currently one of the best.
But he's been there a long time, and it would certainly be a huge upheaval. I imagine he's conflicted, wanting to go and wanting to stay at the same time.