Many reasons. Mindful breathing triggers my asthma, for one thing. I also don't care for the detached aspect of mindfulness - it feels an awful lot like dissociation/depersonalization/derealization to me. I'm also never a fan of throwing judgement out the window. Having compassion for oneself is fine, but it strikes me as rather narcissistic to practice withholding judgement of oneself. And I don't think mindfulness is necessary to achieve self-awareness and be able to separate feelings from reality. At least, I don't find that to be true myself - and I don't think I'm superior or enlightened in some special way. I am prone to valuing facts over feelings, though, so maybe that helps.