Because to is something specific. Perfective aspect is used in a specific event with a specific setting. Imperfective aspect is used in general, non-specific instances.
Oh that makes more sense now, thanks!
But why "Nie mów tego.", then? Isn't "tego" specific?
My explanation about perfective and imperfective verbs applies generally. It's not a hard rule set in stone for all verbs. Different verbs have different meanings, and mówić is not pokazać
I'd treat as something 'general' because it's a bit like Do not say that, ever! I think I saw a 'rule' that negations take perfective verbs, but I'm pretty sure there are exceptions from that.