You need things to go well for you to win a cup. Sometimes you create your own luck, but mostly, it's down to clicking at the right time, and the other teams having a bad day. Any team with money shorten their odds considerably, not to mention being able to field strong teams without affecting other competitions.
The stats are there for all to see regarding cup winners. Yes, a few teams have managed to win, but mostly the winners are the top clubs, and have been for a long time now.