Having more countries will cut the costs of building new stadiums or improving the actual ones by only one nation to having multiple nations that will divide this charge while enjoying more profits from having more games.
It is all commercial business.
But it might also be the case that costs go higher in the case of multiple nations hosting it.
Instead of centralizing everything, and re-utilizing infrastructure, you have to start basically from zero at each country.
Going from zero to one stadium is huge, but going from one to two stadiums is not the same linear increase, it's a bit cheaper, economies of scale.