It can be expensive for people living in parts of the world to follow it because of the way food is priced, according to this. Key quote:
The results indicate that it’s not enough to follow the Mediterranean diet simply by changing the quantities you’re eating of certain foods. The foods need to be of a high quality, too, and you need to eat a diverse range of them.
Both of those things are harder to do on a budget. Fresh produce and fish are often only available at higher costs and in certain areas (this disparity leads to the food desert phenomenon), which makes them harder for low-income people to access and afford.
Everything is harder when you are poorer, including eating healthy.