The time to decide to run for POTUS is at least 2 years before. 2 years ago, nearly all GOP presidential aspirants concluded that Obama would be nearly impossible to beat because he would raise a lot of grass roots money and would have hired a lot of talented people to run his campaign.
They also figured that there would be enough economic recovery to further make Obama unbeatable.
To me, it is surprising that no other candidates joined in after the 2010 midterms. Maybe they figured that the GOP gains were due to the mediscare tactics they used in regards Obamacare and they figured that would not work again if Obama had a chance to respond.
Paul Ryan tried to play that mediscare trick and this time it blew up and he has built a reputation for not being truthful.
Romney was the only viable one who wanted the GOP nomination and he caught some breaks that made Obama beatable, the global economic problems and the impact on the economy due to the debt ceiling debate and crises in other nations' economies.
Even though Obama looked more beatable at times recently, none of the ones who did not run this time are regretting their decision. Their expectations that Obama's team would run an excellent campaign and be hard to beat are seeming accurate.
The problem with the GOP now is that their extremists have more power within the party and the GOP brand has lost its lustre.
Also, independent voters know the recent history of GOP policies and that it is not fair to blame Obama for a slow recovery when the GOP is on record saying that they want so much to defeat Obama that they will put that as a higher priority than taking action to boost the economy.
Where Ignorance is Bliss, 'Tis Folly to be Wise.