MMS has been exorbitantly, insanely expensive and with extremely high failure rates in much of the world until...
... actually that's still the case in varying degrees and locations. And once you've got great market penetration in an area, there's little to no reason to switch unless the competition is noticeably better, which MMS really has no claim to.
Email definitely did not. When WhatsApp was beginning its rise to domination, it was on tons of feature phones - mostly just calls, texts, and WhatsApp. Email implies a lot more general internet access than was generally supported or understood by people, since it supports arbitrary data and hosts.
... actually that's still the case in varying degrees and locations. And once you've got great market penetration in an area, there's little to no reason to switch unless the competition is noticeably better, which MMS really has no claim to.