You can blame the out-of-control ambush activists for that one.  They're ruining town halls, not just for Congresspeople, but for the calm and respectful constituents that are actually there to participate in calm and reasonable exchange.

Town halls have become nothing more than publicity stunts for opposition activists.  The tactic is to make a big scene so that the media sends cameras, puts it all on TV, and embarrasses the Congressperson.  It's more effective than spending 50 grand on a 30-second TV ad.

That's why traditional town halls are being abandoned in favor of more controlled constituent interactions that can't get hijacked by activists for a publicity stunt.  Blame the Tea Party activists if you want for furthering the tactic 8 years ago, when Dems were forced to cancel town halls too.  It's even worse now.