why would i be dropped?

Because the ones you liked couldn't see you as a member of their chaper. Plain and simple.
Why did your least favorites invite you back? They see you have the potential to be an asset in their chapter and want to see if you cold possibly be their new sister.

It's brutal, but it's because they liked other pnms better than they liked you. You did not score high enough to be included in the amount of girls that the chapter was allowed to ask back.
It's disappointing for the chapters too when they invite a pnm back and that pnm likes other chapters better and goes to them instead.
A chapter that wasn't necessarily your favorite can end up being your new home that you fall in love with. Remember, you are going to be part of a great pledge class and will be meeting so many new girls in the chapter too. Don't let it disappoint you too much. Give it a chance.