Simply, Reason being, when any Greek Organization was Founded whether Before 1900 or shortly after, Areas nearest were the easiest to work with and start because of the cost of travel along with "Cant get there from here"!
Example: While The Majority of Fraternities/Soroities were founded East of the Mississippi River, that is where they tried to expand new Chapters! Depending of course on what part of the Eastern Area they were Founded out. North or South.
The two prevelent Starting Places for a Larger Group of Fraternitys were, The Union Triad, in NY and The Miami Triad in Ohio. I beleive there were two places where Soroitys were Founded in Triads in a short period of time! Help Me AGD, and GPB!
One of the Soroitys, not sure off of the top of my Head was Founded in Ark. West of The Big Misissipi!

Either AX or AXO.
LXA was one of the first to Found a Chapter in Cal. at U. Cal. Berkley now. The other that paralleld us was I think AEPi. This was due to The Original Founders of all Orgaizations to Become NATIONAL! National, Border To Border... Next was International, Canada, which is out side of The US!
As time and travel became easier, The Greek Organizations expaned to other areas.
But, in the beginning, time, cost and distance prevailed on expansion.
Greek Organizations are like anything else, Cost, Time, Ability to stay in Contact with and a multidue of things have a lot to do with the moves that are made even in this day and age.!
That is Why, AGD is the only National Soroity in Ha. and that there are so few Greek Organizations in Alaska! LXA being one of the few.