Com'on bro, do you know that in trading you might be very skilled in all this that is listed down here and still be losing money?
That alone entails how difficult trading is, most traders don't just know when to stay out of the market, and that is one of the major cause of losing experienced by veterans traders, trader that have the above skills already.

Secondly, why trading is difficult is that knowledge alone is not enough, you need all the above skills, patience and knowing when to stay off the market for you to stand a chance to be successful in it, and all this skills requires years to masters, but newbies mostly think that it's the fastest way to get rich, not knowing that diving into it with little or no understanding about it is the fastest way to get wreck.