O'Malley had some decent accomplishments (his second term agenda seemed aimed at building a resume to present to Democratic primary voters), but he wasn't particularly beloved. Obama got a higher share of the vote than he did both times, and his lieutenant governor lost to a Republican trying to succeed him.
Also, you could see the attack ads ahead of time: screwing up implementing the ACA exchanges, putting a tax on rain, and especially the Baltimore jail corruption scandal. He would've been a fine mainstream Dem candidate, can't imagine he'd add a lot beyond that.