A. See Talmud Menachos which has different opinions whether a non-wool garment is obligated in Tziztit from a Biblical or Rabbinic standpoint. Bottom line is that a woolen garment is certainly Biblically obligated in Tzitzit according to all opinions, hence the preference for wool. But many do wear cotton Tzitzit, which are Halachicly legit and yes, they are much more comfortable in the summertime or if you work up a sweat, and in general for those who prefer a looser clothing feel.