The Tea Party movement is still about fiscal conservatism. Individuals within the movement however aren't ONLY fiscal conservatives. And the Tea Party movement was NEVER about "being fiscally and ONLY fiscally conservative". As a movement it was neutral on social conservatism and to a point neutral regarding military policy as well. If you wanted to be socially conservative...cool. If you didn't...cool. The MOVEMENT didn't have any real stance on that.
However, individuals can. Its not uncommon for fiscal conservatives to ALSO be social conservatives to some degree. And when those people are faced with two candidates they find acceptable fiscally, then they're likely to go to their other view points to decide which to choose from.
Lets look at an analogy.
You like Pizza. More than that, you like Pizza with Pepperoni on it. That's your biggest concern about a pizza, its GOTTA have pepperoni.
However, you ALSO like Sausage, but you'd not be happy with a pizza JUST with sausage.
If you're faced with a Pizza with pepperoni and one without, its obvious...you choose pepperoni.
But if you have TWO with pepperoni, but one of those two ALSO has sausage...you likely go with the sausage one.
Does that mean Sausage is ALSO your favorite topping? Nope. Its still pepperoni. But when faced with two pizza's that are both acceptable with pepperoni, one with sausage will win out over one without.
That's the case here. Tea Partiers see acceptability in regards to their fiscal views in a number of candidates...so it trickles down to secondary issues for them. Those issues will vary person to person, and area to area.
People may dislike Santorum, I know I am no fan. But when you look at the Contract From America and look at his rhetoric and record...he has a decent claim to being in line with the fiscal views of Tea Partiers. His rhetoric and record is enough to say he's "acceptable" as a Tea Party candidate based on issues alone, not electability or other non-tea party related views, etc. So he passes that first test and moves onto "so what else about him makes me like him more/less than the others who reach an 'acceptable' level for me".