Insecurity.
They get BBL's so they can be seen by more men and feel that they are attractive to men and in comparison to other women.
Most times they did not get a lot of male attention while growing up.
I had one former chick try to run the "I just want a bbl so I can feel good

" nonsense on me and talked about how she wouldn't want to be sexualized by men once she got it, something which she already knew would happen as it's happened to her friends who also got BBL's
What's sad is that these women will get the bbl's and attract unserious men who just want to hit that ass and then leave. And I have zero empathy when that happens as these women already know this is going to happen.