In the "difficult to learn" I am not just including the rules, but how to play correctly. Newbies don't want to play Professor's Research, because it loses them cards. They have a hard time managing energy, which isn't a "fire-and-forget" resource like in other TCGs. And then there is deckbuilding, which is extremely unintuitive. In other games you can pretty much throw cards into a pile and, as long as you follow some simple rules, the deck will work. In Pokemon, the exact opposite is true - if you don't build your deck around Supporters, it will completely stall and you'll look at an uninteractive board. The best example of this are the official Theme decks which are basically unplayable against any half-decently built deck.
The grand majority of players are not competitive netdeckers and they play the game to have fun, not to win at all cost.