I don't think that's a good hypothesis, because in that case, other countries with a huge colonized population such as Mexico or Perú would have less intelligible dialects as well.
Not all Latin-American countries experienced the same level of mestizaje and colonization. The southern part of Chile, in particular, was never successfully colonized by the Spaniards, and mapudungún, the language of the Mapuche people who live there has had (and continues to have) a tremendous influence on Chilean Spanish.
