I think the reason is because of her weight shifting to her back, up on top, under her arms, as she got older. She explains in this Huffington Post article:
Most people acquire money as they get older, Rhonda said with a teasing grin, I acquired cup sizes. I found my weight shifting to my back, up on top, under my arms. So I started creating pieces out of my own necessity. I wanted something comfortable. Dont get me wrong, when I go out, I still bring out the pushup bra to give the girls some lift. But I wanted something that wouldnt pinch and poke me during the day.
http://www.huffingtonpost.com/2013/04/26/up-all-night-star-builds-marl o-thomas-it-aint-over_n_3163955.html