I think it is because the Conservative party is now suffering a deepest ideological crisis as a whole. Some kind of powerful degeneration of the party and their ideological foundations and political self-awareness occurred.
They in essence cannot now offer anything really fundamentally different from what they actively exploited in the last century. They has become kind of ideologically exhausted and turned into a kind of political Dinosaurs the best embodiment of which are such figures like McCain and many others - people from the 20th century and sometimes from the 19th. The best they could think of in this situation is to wrap the old content with the expiry date long gone in a kind of renewed wrapper: and the result was the appearing of plastic dummies like Sara Palin and some others. From the age of the Cold War they - Republicans - still need some kind of war as a pivotal driver and the main justification of their existence in power (not necessarily a military war - just a war of words, a war of measures and ideas): the war with some enemy that wants to deprive the US of its values of its air, water, good traditional family, peace, security etc etc. In this old good scenario they wouldn't have to create any new values, they would just defend the old ones and you would thank and praise them for performing this heroic task.
But the problem is that now comes an age when war is not essential like it was, an age when politicians must be ready to accept new realities and sometimes review some old values. But Republicans just turned out to be absolutely unprepared to generate something new. So there we are: they just mechanically continue to do what the did in the past: search for what could be categorized as bad and dangerous and make an impression of actively defending you against those evils (be it taxes, or Wikileaks, or abortion, masturbation etc etc etc), even if those evils sometimes were unarguably created by themselves (like crisis).
Yes before recent it worked ok and the 9/11 terrorist attacks played into the hands of the Republicans, giving their ideology (or rather lack of it) an extension - a new enemy to fight with, and, there they were needed again. But all too soon people understood that wars in the old fashion breed terrorists faster than kill them, and that a new vision and study of this problem is vital to really decrease the influence and reduce the number of terrorists. But new vision and new studies and approaches require creative work, not heroic slogans or swinging your fists. And creative work is not something Republicans happened to be ready for (not only in fight against terrorism but on all fronts, including international relations, economy etc). So the Party kind of 'degenerated' because of its inability to offer something fundamentally new.
The same goes for economic policy. Republicans have always supported big corporations and tax-cuts for them under the guise of supporting free market and business in general - that's a core part of their ideology of promoting free capitalism. As a result of these warm relationship 1) those big corporations lobby republicans with... of course money and 2) many former and present reach businessmen go to republican party if they decide to go to politics. That's it. Republicans loved big reach corporations (banks, oil companies etc) which in turn love republicans: that was part of their ideology; help the reach, make themselves reach too and then the usual people may have something from all of this too. In this sense they too positioned themselves as defenders of traditional values of free market and entrepreneurship against 'enemy' - all those who ostensibly what to steal those values.
But now, especially because of the crisis, it has been becoming increasingly clear that helping big corporate giants with low taxes has NOTHING to do whatsoever with either support of free market of with support of small and middle-sized business, which are the main generator of jobs and economic development.
No enemy - that's the problem. No body around to blame and to protect us against in order to sell us our air and water in the political sense (we couldn't have these too if it were not for Reps). You just cannot be conservative in the 21th century, because the increasing pace of development of this world is not compatible with 'conservatism' in the shape it was viable in the last century. The whole ideological basis, or, to be more accurate, and illusion thereof, has just fell at the seams