Thanks for the answers! I guessed it might be most of the reasons stated and figured in the end it all really boiled down to personal preference and what works in your design.
And I'm 100% with whoever stated that cotton 'breathes' (and absorbs) better and is much more comfortable for summer clothing! Had on a tank top this weekend in an effort to stay cooler while working outside. Forgot it was cotton and spandex instead of just cotton. . . Won't forget again!