For us big-boobed gals, a sports bra is not a simple, singular piece of cotton spandex. To ensure even the most basic support, we require underwires, numerous back clasps, double-layered cups, adjustable padded straps, levers and pulleys, etc.
See this basic sports bra here? See how the racer back puts all the weight of your breasts around your neck? Put me in one of these things and have me take a leisurely walk around the block -- I'll come crawling back looking like Quasimodo and begging for mercy. Awful.