It is a nice idea to organize the World Cup in different countries. But in terms of transport, it is not optimal for the players. You will always have to travel up and down, which is not great for your preparation. And the distances between these countries can also be considerable. Mexico and a World Cup, we've already experienced that once before in 1986. I thought everything had gone pretty well then. It surprises me a little that they organize the World Cup, since America is not really a football country at all. At least, football is not among the 3 most popular sports. Maybe not even in the top 5.
When it comes to FIFA you only need to think about the money to find out why they do things they way they do, an increase in the number of teams means more matches and more money for them, however since this is the first time they are changing the format of the tournament in such a long time they are trying to distribute the burden among several countries and see how this experiment turns out, also even if the US is not a country that has a long soccer tradition, FIFA has tried for a long time for this to change as there is a lot of money for them to make if this changes, so they are trying to improve the popularity of soccer at the US with this move.