Additionally, Ben's powers were fading on the material plane. IIRC, it was David Gerrold who wrote in Starlog
that he thought that Ben foresaw his own death, as evidenced by these lines [according to http://www.blueharvest.net/scoops/anh-script.shtml
LUKE: I think my uncle knew him. He said he was dead.
BEN: Oh, he's not dead, not...not yet.
There's something to be said for that interpretation, and additionally, there's this:
BEN: They must be delivered safely or other star systems will suffer the same fate as Alderaan. Your destiny lies along a different path than mine. The Force will be with you...always!
Of course, there's also this:
VADER: Your powers are weak, old man.
BEN: You can't win, Darth. If you strike me down, I shall become more powerful than you can possibly imagine.
If it was Ben's fate, then the Force would deny him the strength to alter his destiny, should he try to do so.
In the PT, in ROTS, Yoda sets him up to ascend and guide Luke from beyond, which further reinforces the idea that dying on the Death Star was Obi-Wan's destiny.