Can you just imagine what it would be like if the Republicans decide to put their 2016 national convention in Dallas? Especially if the state's current junior Senator and reigning Junior Tailgunner Joe McCarthy enters the race and is still in it at that time?
Shades of Adlai Stevenson in 1963, will we have tea partiers spitting on the "librul media"?
That said, as Gillman notes, although his train of thought doesn't go quite that far, this is probably why the Republican National Convention wants to avoid Dallas 2016 like the plague.
As for Cruz running? Like Dear Leader, but for totally different reasons, I think it has to be now or never. No incumbent, unlike 2020, and as short a time in the Senate as possible.
In Obama's case, it was to avoid being caught in Senate inertia and minutia, and becoming more clear to more people as the semi-hopeless neoliberal that he is. As for Cruz, it's to avoid batshit-crazy burnout. Let's say that a Republican is elected president in 2016. First time the debt ceiling comes up and he filibusters against his own party, the last bridge is burned.
C'mon, you'd love to see Cruz on national TV as a modern, nuttier, Barry Goldwater on some combination of roids and speed, wouldn't you?