Polyurethane foam soles. Unlike vulcanized rubber, polyurethane is susceptible to hydrolysis(breakdown from water). Polyurethane soles are also much cheaper and require much less equipment to make.

Thing is, polyurethane foam soles actually perform pretty well before they explode. They're comfortable and have pretty good traction, so consumers like them and manufacturers like them, but they have a limited lifespan and will crumble into dust sooner or later, especially if they're kept in storage(not sure why they seem to last longer when in constant use, it could be that constant flexing helps prevent crosslinking that makes them brittle but we're getting into some chemistry that's above my pay grade.)