So even though the recent freshman congressman are on average from more competitive districts, on average they are to the right of the republican incumbents they joined. The GOP took a really hard turn to the right over the past three years.
I'm not sure it's so much that the Republican party as a whole, especially the "institutional" level, took a hard turn to the right as that it's found itself between two pressures it never expected to face. One is the 2010 crop of newbies, who got elected campaigning on old conservative meathooks like absolutism on abortion and such, and the Republican establishment never got around to explaining to them that it's
supposed to be an act. That they're supposed to campaign on abortion and gays and prayer in schools and crap to get elected, then leave all of those issues as legislatively ambiguous as possible so they can still use them in the next election.
Instead, they wound up with a crop of zealots who've been drinking the koolaid for so long, they don't know how to leave things be. Partially because, in the year 2010 of Citizen's United and Obamacare, the conservative think tanks and advocates who always funded the Republican party, your ALECs and Family Research Councils and so forth, suddenly had unlimited money to throw at candidates, and a bumpercrop of true-believer nutcases to pick from. Many of those groups really do believe the stuff they push, and have been waiting with the patience of saints for the party they've stood behind for thirty years to finally do what they've been paying them to. At last, an election came along where they didn't need to deal with the Republican party structure at all, and got the loyal soldiers they wanted, and told them they were invincible.
And now, since the Republican presidential candidates believe they have to hitch a ride on this train to win, they're right there with them. That's how you get candidates like Rick Santorum sounding "reasonable", because the 2010 alumni have made it common knowledge in Republican circles that abortion is so old hat it's time to discuss the moral legality of contraception. That's also why you don't see the other Republican posterboys like Chris Christie running this time either, because they know that train is going to derail sooner or later.