I think that when you use the term "conservatives", you are referring to the right wing. Today, Reagan would be considered a mainstream or establishment republican, given some of his history as well as his penchant for working with the other side. As a so-called conservative, he signed numerous bills which increased the deficits and debt; signed real amnesty, walked away from Lebanon, frequently lunched with Democrats, and when he accepted his party's nomination, mentioned only women's rights as a social issue. And I'm a Reagan fan.
The disasters of those you mention have a lot more to do with Republican inability to conduct strong campaigns than the conservatism of the candidates.
I wish those on the right side of the right side of the aisle (no I didn't repeat) would understand what conservatism is. Romney, Bush, Rubio, and yes even Christie for the most part are conservatives.
I don't disagree that Reagan governed as a moderate. But he was not afraid to champion the conservative cause, and to hold himself up as a symbol of how a conservative should be and live. For that reason, conservatives are generally willing to overlook a lot of his moderate policy initiatives.
I'd have to stretch my imagination a long way before I could swallow the idea that Romney, Bush and Christie are conservatives. That doesn't preclude my possibly voting for them in a two way race, but if you put these three under the umbrella then the definition of conservative has no real meaning. I'd agree that Christie is a fiscal conservative, but conservatism encompasses much more than that.
BTW, I don't even consider myself a conservative. When people ask me what my political orientation is I usually call myself a small l libertarian, or a constitutionalist. I don't think Rand Paul is a conservative, either. I do think Rick Perry, Ted Cruz, Jeff Sessions, Rush Limbaugh and others of their ilk are conservatives.
During the last campaign, Romney himself would not call himself a conservative, only that he applied conservative principles to his decision making. GWB would not call himself a conservative, either. He had to soften it with "compassionate." In my book, if a politician is not willing to stick the label on himself, it's a pretty doggone good bet that he or she is compromised in some way.
All that said, I think conservatism as an ideology is a loser as a proposition in a general election in the USA. There need to be more people brought into the tent than free market advocates, patriotic and religious types. All three of these are shrinking constituency groups. Traditional moderation won't do it, either. People simply won't trust it. Only a true reformer can win against the Dem machine.