2007 was a different time. (A time when almost nobody had smartphones - how foreign is that from today's perspective?) Have a look at the plan available in the US for the iPhone at the time: https://www.apple.com/newsroom/2007/06/26AT-T-and-Apple-Anno...
Even the cheapest plan has unlimited SMS and effectively unlimited MMS as a result. Whether it's used more because it's included or whether it's included because it's used more (probably a bit of both)... I don't think 2007 is a useful reference point for what people expect from their phones today.
The cheapest plan on that page is $30. I get unlimited MMS on my $17 plan in Europe, so I don't see how the pricing would make MMS more prevalent in the US than in Europe.
Do you have a reference supporting the idea that MMS is mostly relevant to the US and not the rest of the world? It doesn't match up with my experience, but maybe my family and friends are atypical.
200 SMS messages (MMS counts here too), unlimited data. Now look at today's: https://www.att.com/plans/wireless/
Even the cheapest plan has unlimited SMS and effectively unlimited MMS as a result. Whether it's used more because it's included or whether it's included because it's used more (probably a bit of both)... I don't think 2007 is a useful reference point for what people expect from their phones today.