Lack of enthusiasm, elitism perception, awkward debater at times, seemed too fake. So many reasons. Obama won by default. He didn't run a great campaign in '12.
Except that he did. Except for his debate performance, name one flaw.
The biggest mistakesThe 47% gaffe
Letting Obama define him as an out of touch corporate vulture rather than defining his business record positively
His many gaffes that made him look out of touch 'I don't care about poor people'
Record of flip-flopping
Failure to appeal to demographics like women and Hispanics(and not compensating for losses elsewhere)
In addition there were plenty of smaller mistakes within them and altogether his campaign was no match for Obama's.