This has been brought up before several times over the years and the answer is always the same: it's a complete abandonment of logic for someone to have an opportunity for ANY championship and not challenge for the WWE World Heavyweight Championship. Vince is accused, rightfully so a good deal of the time, of glossing over huge gaps in logic with such casual ease that it gives the impression that if he doesn't consider logic important, then neither should the fans.
The simpler and more logical method would simply be to have a Money in the Bank ladder match in which the winner has the opportunity to challenge for either the Intercontinental or United States Championship at any point over the course of a calendar year. It'd be a way of putting more emphasis on the mid-card titles, though WWE has been doing that over the course of the past several months. The US title has pulled ahead of the IC title in terms of prominence, though there's a strong possibility of them being on an even playing field had Daniel Bryan's injury issues not resurfaced. Still, at least they're not playing hot potato with the title, they're not jobbing Ryback out left & right and have applied more general prestige to the title than we've seen in quite a while. As for the US title, you have to be a complete and utter moron to say that Cena hasn't brought the title to a different level, one that hasn't been seen for the US title since well before WWE reactivated it back in '03.