Why did the Istari take the form of Men specifically, why not another race?

I get why the Istari were given old forms, so that they would appear weak and humble in order to guide middle earth and not to rule it, but why was it Men specifically?

Could not one or more of them have been given the appearance of a old looking Dwarf or Elf?

Gandalf specifically was often mistaken for being an Elf.