why do southern sororities only want southern girl

Tradition. It's always been like that. It worked in the past because they weren't that many oos students who rushed. Now it's beginning to hurt the old row type houses because they're losing such great girls to new row houses.

The funny thing is that many sororities at bama rush based on what their last year's pledge class pushes for. So it is a high school popularity contest. Layer onto that a network of moms and alumnae that press really hard for a family member or close friend to get a bid. What happens is certain houses have so few spots that are truly open. There are increasing oos pnms and so what happens is many of the great oos pnms go "new row" where they won't have to deal with all the Alabama school girl crap. This definitely happened in the last cycle where several new row houses (many of which are national powerhouses) are seen as having top flight classes while more traditional in-state focused houses are seen as having a real mixed bag.

First of all, check any sorority (well, if they posted the bid lists you could). ALL the sororities are taking loads of OOS girls. NO sorority is exclusively all-Alabama women. Yes, there are some with classes of 50% Alabama - and why no? The instate women are more likely to marry Alabama men and stay in state, close enough to help their chapters.
ANY sorority in ANY state will tell you that.
Second, can you guarantee that a Southerner going to a Northern school would not be driven to distraction by the dumb questions and assumptions, being made fun of, and not being taken seriously by the locals? No, didn't think so.
I don't understand why Yankees are so fearful of being rejected by the local sorority women. If you're that scared, why come here?

I am from the north and did not feel like any house had only southern girls or that i had less of a chance being from the south. Well, at all houses except just a couple of the "old row" but really it was not bad.

Same reason some northern sororities will favor instate pnms- in my opinion ....either they
1) they know them personally already
2) are from same region, so easier to make conversation or connections-oh you went to XYZ, did you ever ABC- attend concert/big rivalry, etc... and
3) transfers- the further away from home you are the more likely you are to leave/transfer etc which hurts retention