Because the mind grows, wanders, and sees the outside world. Hence, the mind wonders, asks, and opens itself to other possibilities.
If ignorance is bliss, why aren’t more people happy?
Maybe because in their chosen path of ignorance, in the hopes of keeping their choice of status quo, they can’t experience true freedom.
Maybe because in keeping themselves ignorant, guilt is slowly eating their hearts, their sanity, their morality?
The guilt of knowing the truth but keeping it hidden, because in keeping the truth they get to maintain their choice of status quo.
Thus, with guilt building up, ignorance is not bliss, ignorance becomes a badge of shame.