Historical Linguistics and Language Families 9, Exercise 2: creoles and the tree

Two reasons (Wikipedia, Creole language):
  1. The tree model assumes regular sound changes; creoles do not fit it.
  2. Classification usually relies on inherited core vocabulary and grammar.
     In creoles the core lexicon often has mixed origins and the grammar is
     largely original, so there is no single line of descent to draw.

Monogenetic theory: traces Atlantic creoles to a single Portuguese-based
pidgin. (The page also gives the European dialect origin hypothesis, which
treats French creoles as phylogenetically based on French, and a view that
"creole" is a sociohistoric rather than linguistic concept. The page says the
question of which language is the parent often has no definitive answer.)