The widely appreciated reason is that, if Romney's foreign policy team winds up looking anything like his current team of foreign policy advisers, it will
include a fair number of neoconservatives. And what American enthusiasm there is for intervention in Syria resides largely in neocon circles.
"Intervention," actually, could wind up being an understatement. Some neoconservatives see Syria as a gateway to Iran, which means their version of
intervention could wind up broadening the war rather than hastening its end.
The second, largely unappreciated reason a Romney presidency would raise the chances of intervention has to do with the one other place in America where
there's support for a Syrian intervention: among liberal interventionists. Some of these liberals have been biting their tongues on this issue because
they're Obama supporters and don't want to be seen criticizing Obama, especially right before an election. But if Romney wins the election, you can expect
them to pipe up. (I stole this insight from Robert Farley, who uttered it on the
BhTV show Foreign Entanglements, which he co-hosts with Matthew Duss.)
In other words: Come late January, we could have a neocon/liberal-interventionist coalition supporting intervention, with neocons well-represented in the
White House. Sound familiar?
Possibly mitigating circumstances:
 There's a chance the neocons would lose the struggle for a Romney presidency's soul and some of Romney's more moderate advisers, such as Bob Zoellick,
would dominate foreign policy. (Though even Zoellick signed one of the early Project for
a New American Century letters that were part of the neocon campaign that culminated in the invasion of Iraq.) In that case chances of involvement in Syria
 The Assad regime could well fall by late January. Even so, given the significant chance that Assad's fall would be followed by continued fighting along
sectarian lines, the situation could long remain sufficiently chaotic for arguments in favor of intervention to persist.
I want to emphasize that I don't consider intervention in Syria as obviously bad an idea as bombing Iran. Syria is a tragic mess, and, though intervention
doesn't seem hugely auspicious, neither do the alternatives. With Iran, in contrast, there are clearly superior alternatives to military action. (One of
these is a negotiated deal--a deal that, as I've argued, a first-term Romney administration would have a harder time pursuing than a second-term Obama
administration. The other option, in the event that negotiation fails and Iran does choose to develop nuclear weapons, is containment, a policy that both
candidates reject, though it seems pretty workable to me.) But, whatever the
merits of intervention in Syria, I think it's more likely to happen under Romney than under Obama--not just because a Romney presidency could empower
neoconservatives, but also because it could give voice to liberal interventionists.