As Mitt Romney auditions potential candidates to join his ticket as the vice-presidential nominee, a cadre of Republicans have been championing one name as a no-brainer: Florida Sen. Marco Rubio. So when ABC News reported Tuesday that Team Romney has not even asked Rubio to complete questionnaires or financial disclosure forms — all but confirming that he isn't even being vetted for the VP gig — many political junkies were shocked. "Young, charismatic, and wildly popular with conservatives," the Tea Party-backed Latino was once widely seen as the VP frontrunner, says ABC News' Jonathan Karl. Why is Rubio now reportedly off the short-list altogether? Here, five theories:

1. Rubio's inexperience would hurt Romney 
Rubio wasn't sworn in as a senator until January 2011, says Alex Altman at TIME, so bringing him onto the ticket "would undercut Romney's claim that the country made a mistake by trusting" the similarly inexperienced Barack Obama in 2008. Romney would face no such problems by picking "a safe, experienced running mate" with a longer track record in politics.

2. He's been tainted by allegations of impropriety
An "aroma of ethics scandals... occasionally wafts from" Rubio's direction, says Ed Kilgore at Washington Monthly. That includes past instances when he used Florida GOP credit cards for personal expenses. But what's especially concerning is Rubio's close relationship with Rep. David Rivera (R-Fla.), who has been investigated by the FBI and IRS over accusations that he abused his office for financial gain. Indeed, "you can sum up" Rubio's "biggest impediment to being chosen vice president in two words," says Chris Cillizza at The Washington Post: "David Rivera."

3. Romney may have made up his mind already
With Rubio off the list, one could surmise that Romney's "already leaning in the direction of one of the white-bread alternatives," says Kilgore. Ohio Sen. Rob Portman may have sewn up the slot already. Or perhaps former Minnesota governor (and ex-Romney rival) Tim Pawlenty has. Indeed, chatter surrounding Pawlenty "has picked up considerably in recent weeks," says Doug Mataconis at Outside the Beltway

4. Romney is writing off the Latino vote
Everyone assumes that Rubio would help Romney shrink his massive deficit with Latino voters, says Josh Kraushaar at National Journal. Yet Romney has spent much of his recent six-state bus tour in the "white, working-class Rust Belt, hitting small towns without much of a Hispanic presence." Winning over white voters in key states like Ohio, Pennsylvania, Michigan, and Wisconsin is clearly becoming just as important to Romney as appealing to Latinos. A running mate with "working-class appeal" like Sen. Portman could be what he needs most.

5. Hold on. Rubio could still be vetted
"Check the calendar," says Ed Morrissey at Hot Air. "It's June." Romney's campaign has plenty of time to look further into Rubio as a potential running mate. Perhaps, but it's been nearly two months since Romney aide Beth Myers was hired to head the vice presidential search, says Karl. That Rubio hasn't gone through the process yet is a strong indication that he won't — ever.  

Editor's note: After this story was published, Romney insisted to reporters that "Rubio is being thoroughly vetted." Separately, an unnamed Romney adviser told The Washington Post that ABC's report is correct, and Rubio has yet to be vetted.

Read more political coverage at The Week's 2012 Election Center.