Not so much luck as just too many qualified students applying to the hyper-competitive schools. I thought two observations put it in perspective.
I visited one of the very top-if not top–LACs with one of my children about six years ago. In the info session, rep said that 70% of applicants were fully qualified academically, and it appeared that they would be great people to have in the school’s community. At the time, the LAC’s acceptance rate was 14%. So only 1 of 5 fully qualified students were accepted.
A few years ago, an education writer asserted that Harvard, or a school like it, could go through the process, select its incoming class, throw that class in the trash can, choose a whole new class, and there would be no discernible difference and the predicted success of the new one would be the same as the old one.
So if a student is qualified, it is somewhat a matter of just having a little something somewhere in the application that strikes the right note with the people reading it.
I’ll note also, it must be really difficult being on the adcom at one of these schools, making many such fine distinctions to come to decisions that mean so much to those who applied.