A lot of people mention the cost of MMS, but at least for the American carriers I'm familiar with, MMS comes out of the same bucket as SMS, so if you have a text messaging plan, there's no extra cost unlike if you wanted to have email/internet access. The average consumer is much more likely to spring for an SMS add-on plan (cheap and extremely commonly used) than a data plan (tends to be expensive and not so ubiquitous), so to quickly send pictures using MMS just makes sense for them.

About some people not being able to receive those messages: nearly every dumbphone and featurephone is MMS-capable, no matter how cheap and aged it might be. The same is not true for email.