Because they will see that as the only scenario which will keep together the two factions.
Obama might be ego driven, but he is NOT that irrational.
He will succumb to the massive pressure from the DNC and Kennedy and all the other Democratic powers-that-be.
After all is said and done, the Democrats desperately want to defeat the GOP more than they want to hold onto their petty differences....
Obama will swallow his pride in order to keep the Hillary faction from defecting to McCain. (plus, Obama will likely hold out hope that Hillary could be scandalized or impeached.... which would thrust HIM to the Presidency)
Obama is still young, and if he ever wants to be President, he'll take whatever deal he's offered.