The ETRTO recommendations are rather odd, in that they like to mount really fat tires on really narrow rims.
They offer no explanation of this, at least for free.

The general trend is towards using wider rims for narrower tires. The prime example of this is fat-tire road bikes that have rims AND tires that are both around 30mm wide....

For best results, you don't want your inflated tires to be greater than about 125% of the rim widths.

Wider rims allow you to use less inflation pressure for the same width of tire, which provides better traction while providing a more comfortable ride than you would get with narrower rims.