Seems like a question best answered by the author(s) of BIP32.
Other than that it is the age old question of “how much entropy” do I need?!
This is a pretty interesting read re: 128 bits in general.
Also as a rule of thumb: when you insert 128 bits of entropy in a system, then it also forms the upper boundary for the entropy of anything derived from it (with the exception of key stretching).
If I’m allowed to speculate the intent of the author, 256 bits is probably advised because the eventual keys have that exact size, and that way the entropy matches the keysize.