I think when rush ocurs plays into this a lot.
At my school, rush ocurs at the beginning of 2nd semester. A lot of girls will have formed their friend-groups by this point. Unfortunately my school is very stereotypical when it comes to Greek life--i.e. these type of girls will go here, these will go there, and so forth. So these clumps of girls will join sororities together and so a large part of the pledge class are these groups.
Luckily for me, my lovely ADPi had a unique year. We just rock that much

Where as the pledge classes (err, excuse me, Alpha classes) above our's are mainly groups of girls who are characteristically "ADPi" at our school, my Alpha class is different. We have 46 beautiful and absolutely uniquely amazing girls, who fit into every sorority on campus (as far as "stereotypes" go). For whatever reason, we really got lucky my year. The plus side of this is that we are all friends

And we all get along together well. There are no "cliques" in my pledge class. I count my lucky violets for that.
My roommate's sorority, on the other hand, is not so lucky. There are around 10 groups of 4-5 girls who are pretty exclusively friend groups/cliques in her pledge class. They are an odd assortment of cliques who don't seem to really get along. I feel bad for them ;( but in the Panhel spirit, hope things get better.
While girls rushing together could work out well, let me tell you a sad story. I rushed this past spring (I'm a sophomore) with seven close friends. The eight of us were set for Greek life--we already attended a lot of fraternity functions, etc. etc. A sophomore I knew well was in a sorority (she is now my big sis!) and I knew the girls there were cool; as well, my friends and I however really bought into the "good" sororities on campus.
Every single one of my 7 friends dropped out of rush. The first was cut by the 2 sororities they most wanted; slowly by pref night the rest dropped out. One of my friends actually got invited back to ADPi for Pref Night but her ego was so bruised from being cut by "the #1 sorority on campus" that she dropped out and didn't sign pref card or anything. One of my friends later (after dropping out) tried to COB my sorority, but she didn't get a bid [we had something like 30ish girls try for 2 COB bids].
It's really sad. Everytime I have a Greek function or a sorority sister waves to me on campus, I think they feel even more isolated from the Greek life which flourishes here. The social scene IS dominated by fraternities and sororities and they are very involved socially, they just don't belong to a sorority.
I have asked my friends if they wanted to re-colonize a chapter that "died" on campus within the past 10 years (notably KD and ZTA, which have a strong alumni base & great local chapters 6 miles away at UNC-Chapel Hill) but they don't. They are so caught up in being in the "Best" sorority they won't give anything else a shot.
ADDED TO SAY: Sometimes I wonder if we had first semester rush, they wouldn't have rushed all together and almost collectively decided to drop out. I know some girls would have gone certain places had they stayed in.