Why is ignorance not bliss?

Because what you don't know is still real anyway, so if you are not aware of it, it can hurt you.

Forewarned is forearmed.

Unless you are driving the train, or can contact the driver; Knowing that there is a broken bridge comming up, would be bliss compared with knowing about it and being filled with dread. That is one of the reasons as to why ignorance is blibb; otherwise its just ignorance!

That can only be seen as true if everyone on that train is ignorant and "believes" that ignorance is bliss. People who do not would rather know that their lives would soon be ending. Also as you set up a situation that you believe favors you idea let us try one that favors wisdom. Say you had an argument with someone you loved, many would wish to know of the upcoming accident so that they may spend there final moment telling them how they truly feel before their end comes.

The ignorant believe ignorance is bliss, The wise do not the difference being that the wise spend much more time pondering what is true.

I believe this is a misconception based on the idea that knowledge increases sorrow (whether true or false), but knowledge and wisdom are not the same. One who learns may discover things that are unpleasent and wish to forget them but the wise can put them into perspective and benefit from it whether pleasant or unpleasant.