1) they think it's too much like sci-fi, or that its flavor is to pseudo-scientific;
2) they don't think it has any place in a medieval fantasy RPG, because it has no significant basis in European myth and legend, for the same reasons such folks dislike oriental classes like the monk;
3) they believe that the psionics rules are too broken and unbalanced, to the point of not even giving them a fair chance in the first place, in some cases just because of bad experiences with previous editions of the rules;
4) they don't want to learn a new sub-system, no matter how much it is or isn't like the core spellcasting rules, and they think it's just too complicated on top of already having to remember the combat rules and spellcasting rules;
and/or 5) they don't like the flavor of psionics as it stands right now, and can't be bothered to just accept an alternate description of the powers and their origins, like one of the several alternate flavors of psionics that have been used by lots of groups already, and even discussed on the forums occasionally.