I'm very busty and I would never wear a bandeau (or triangle) bikini. They have absolutely no support; I would be "falling down" if I attempted it. I can barely wear a strapless bra. I even fell out of my regular halter bikini top and I tied it very tightly. If I ever get my bikini bod, I'll probably have to splurge on something with underwire.
Because of the lack of support. Bandeau bikinis are awful in general, especially if they don't have straps. Then you just end up having to pull it up or hold it in place so that it doesn't slide off your body (regardless of what size your boobs are to be honest). Also, for women with bigger boobs, it just looks like the boobs are going to fall out any second because they're so squished together in a bandeau.
Because bandeaus want be able to support the natural weight of bigger breasts. A lot of the bandeau get tucked under and rather than help smooth out the look d breasts, it makes it look like the breasts are sagging. A-C cup women have no problems when it comes to bandeaus. It seems to suit them. D cup onwards, there's only this need to pull the bandeau up and trying to prevent it from sliding under the breasts.