why are they bottom tier?

If we’re going by ranks (which I think is stupid & doesn’t matter), I honestly think that PM and XO are “mid tier” and SK is the only “bottom” one but the reason they’re viewed as the lower 3 is:
SK is the newest sorority and not well established so they can’t be picky during recruitment and as a result have a good amount of girls people consider unattractive but I honestly think they are one of the most genuine
PM has had mean girls in the past and as a result people have not wanted to join and they had to be less picky too but in the the last few pledge classes & past couple years they have started to turn their reputation around, been more involved and have a good amount of pretty & nice girls
XO didnt go out to parties as much and wasn’t really involved in other people’s philanthropies but like PM in the last few pledge classes they really have been making a name for themselves and won a few philanthropy weeks and they’ve been doing great and they have a good amount of pretty & nice girls too
I’m guessing you’re a PNM so one tip is don’t pay attention to the “ranks” I know girls in all 3 of these sororities and they all love their houses and they have a strong sisterhood and I also know girls who picked “top houses” just for the name and were not satisfied so they ended up dropping cuz it wasn’t what they were expecting you just pick where you feel most at home in the end these 3 plus the other 4 may have pros & cons but they are all amazing chapters :)

Its usually based on return rates. If no one wants to go PM XO or SK during rush then they are at the bottom. Its an easy concept. ZTA and AXO are top because most people want them during rush, and they get to choose which girls they end up with