You answer:

For me personally, it's because the learning curve is far too steep. As it appears with most card games, people have either had years of experience with them and are able to play, or have the cognitive strength to understand them almost immediately. Then there are people like me who can't understand them to begin with. Poker, Rummy, Solitaire, Spades etc. seem like an enigma of complexity and obscurity and I suspect it would take several years at least for me to grasp one of these games