Douthat points to his debate performances:
[Perry] was fundamentally incapable of exercising the office of the presidency. We don’t elect a debater-in-chief, but the idea, floated by George Will of all people, that debates “test nothing pertinent to presidential duties” is equally false. They establish a minimal threshold that any politician seeking an office whose chief weapon is often the bully pulpit needs to be able to clear.
Buzzfeed Politics says, simply enough, that "running for president is hard":
Perry could have spent a couple of years as Barack Obama did: Using his elected office to conduct rolling seminars with policy experts; developing a years-long plan for national office; carefully picking the national issues with which to engage. Instead, Perry got into the race on what amounts to a lark. He leaves it badly damaged, limping home to Texas where he'll struggle to regain the clout and swagger he projected six months ago.
James Antle III partially blames campaign staff:
At times Perry's team appeared divided between people who were loyal to him but didn't know how to run a national, as opposed to statewide, campaign and those with a better feel for national politics who were less invested in Perry personally. The end result was that the Texas governor who once looked like the likely Republican nominee ended up underperforming Fred Thompson at every juncture.
Larison concentrates on Perry's foreign policy flubs:
What hardly anyone anticipated was that Perry’s candidacy would make George W. Bush look good by comparison. No one would have confused the original candidate Bush with a foreign affairs expert, but even Bush at his most ridiculous in the 2000 campaign never blundered so badly on foreign policy questions
Jonathan Bernstein wonders whether Perry reminded voters too much of Bush:
Perhaps it was the memory of George W. Bush — before Perry entered, a lot of pundits (but not me) said that there was no way another Texas governor would be nominated so soon after Bush, and perhaps there was something to that. Perhaps Perry's gaffes would have been excused a little more easily if they didn't remind people, somehow, of what happened the last time Republicans decided that policy knowledge was irrelevant and nominated Bush.
And Alex Koppelman thinks Perry underestimated the GOP base:
[H]is failure can be taken, in some way, as a positive sign for the country. As soon as he started his campaign, it was clear Perry couldn’t win by trying to seem Presidential, much less intellectual. So instead he ran as the pure Republican id: the simplistic proposals (part-time Congress!); the silly rhetoric (“I’m not ashamed to admit that I’m a Christian, but you don’t need to be in the pew every Sunday to know there’s something wrong in this country when gays can serve openly in the military but our kids can’t openly celebrate Christmas or pray in school”); the full-bore Texan shtick.