Sean Bielat isn't the type of candidate who could win a congressional race here, even if he wasn't facing a Kennedy.
I don't really see how he's particularly unelectable by the standards of MA Republicans. Of course, it'd be a stretch for anyone with an (R) after their name to get elected anywhere in MA.
(Though the MAGOP is so fail, topping off not even running candidates in over half the state house races with nominating a child molester and a birther in their two top congressional pick-up opportunities).
Granted there's not an especially large pool of successful of folks to choose from, but successful candidates for congress have always come from the State Legislature or had some other history of past office. It's especially important in a state like Massachusetts to have a solid base of voters in your corner before running for office.