Actually... no... Dubbing houses try the best to find voice actors for the characters. The Simpsons producers even went to the dubbing house to cast people, so basically they were chosen by the original creators.
The full story is this one:
The original dubb was done by the dubbing house
Audiomaster 3000; when it closed the dubbing was held to be worked by
Grabaciones y Doblajes Internacionales, that still used the same voice actors (different company, same people) and which is affiliated to the
ANDA (
National Actors Association by its spanish acronym). A contract with the
ANDA stipulated that only voice actors affiliated to it (
ANDA) could be hired by
Grabaciones y Doblajes Internacionales. Before Season 16 of The Simpsons,
GyDI broke the contract and couldn't get with a solution with the
ANDA. So the original voice actors were fired (since they were affiliated to the
ANDA) and new ones were hired.
Actually, Humberto Vélez (Homer's original voice) and Gabriel Chávez (Burns' original) started a boicot against the new dubb. They got it to the point they were told they were going to dubb the movie, which sadly, didn't happen.
Not a full list but this is the dubb actors of the Simpson Family:
* Homer Simpson: Humberto Vélez (seasons 1 to 15) / Otto Balbuena (since season 16).
* Marge Simpson: Nancy McKenzie (seasons 1 to 15) / Marina Huerta (since season 16).
* Bart Simpson: Marina Huerta (seasons 1 to 9, the again since 16) / Claudia Mota (seasons 9 to 15).
* Lisa Simpson: Patricia Acevedo (seasons 1 to 15) / Alexia Solís (since season 16).
Here's a video comparison, of course, you wouldn't get it if you don't listen to the spanish dub all the time xD