Some cards, like Voidslime, counter abilities - but they say they can't counter mana abilities. Why is that?
Because mana abilities get special treatment that prevents countering. They are abilities which resolve instantly, without first going onto the stack. Because of this, there's no possibility to target them with any effect before they resolve, and they can't be countered by a counter spell.
Note that Voidslime's italicized "(Mana abilities can't be targeted.)" is reminder text. It's a statement about how the rules work in general, not a specific limitation of Voidslime.