The time really wouldn't have been right for Trump in 2012. Anger at immigration hadn't boiled over, ISIS wasn't on anybody's radar, & the general feeling was that things weren't good but they weren't getting worse. That had changed for many of the people who mattered (& propelled Trump to victory) in 2016. Had he ran on the same populist platform, he would've lost the general by a significant margin.
And the birtherism thing was also something that most people, regardless of their views, thought was ridiculous & wouldn't get behind. Birtherism had a peak, in mid-2009, as the Tea Party rallies got a bit out of hand, but afterwards, it was widely discredited & even the most populist voices on the right (like Limbaugh & Hannity) were calling it out as dumb.