Because random passers-by did not consent to seeing your naked body in a public space. A burkini is no different to simply being fully clothed at the beach, with the benefit that the wearer can still go in the water. It's also not largely different from wearing a wetsuit at the beach, with the exception that it is less revealing - which regardless of religious views, some women may feel more comfortable wearing less revealing swimwear at the beach due to e.g. not feeling confident in their body shape or size.
Nudism isn't a solution to this either, as there is no reason a woman should be required to wear something more revealing if she wouldn't feel comfortable doing so, even without societal pressures about body image. In fact there is no reason to prescribe what a woman does or doesn't wear (or a man, but given the long history of misogynstic control of womens bodies by men, the former is more immediately relevant).
Going to a nudist beach or similar that is private, and separate from the main public area and not going to have random people who may not WANT to see your body hanging out being exposed to that is perfectly acceptable. But it is really the most insufferable conceit to suggest that someone being forced to view your naked body against their wishes is their fault, and not yours.
Really the comprehension of consent on this website, both from sexual (rape, obviously) to completely non-sexual situations (such as the nudism issue above), is appallingly low.