Last season I was L10 and I was in a L13 Cup. Now I find L14, and of course I just lost on just the second round. I will probably never win.

Why is the cup so different from the other 2 tournaments? Why must it be so hard without any reason?

Judging by team level may be wrong, but considering that you can lose by much weaker teams for no reason whatsoever it doesn't make any difference.