If you're dying to hear opinions, it's one of the mostly hotly debated topics, you can dig through past threads to read millions of possibilities.
To save you time, I'll try to list the ones I've seen a lot over the years.
• Well-played pool is dull to watch, and if nobody's watching, sponsors won't pay to advertise.
• The players aren't professional and get away with acting or dressing in a low class way.
• It has negative image because the media portrays it as a pastime for con artists and barflies.
• There have been various specific scandals that make sponsors avoid it even though it's popular.
• The few who try to promote pool in a serious way might be great players but lack business sense.
• Pettiness, greed, and corruption (both the players and the 'management') have held it back.
• Inconsistent equipment and rules make it seem more like casual recreation than a "real" sport.
• The promoters are out of touch with the everyday player (e.g. they don't promote 8-ball)
• Because bars put in pool tables, it's permanently associated with smoking, drinking, and fast living.
• Players are not 'telegenic' or 'athletic', you can master the game despite being morbidly obese.
• Many pros are moving to poker, and fewer pros = fewer people to look up to or promote the sport.
I could probably come up with more if I really wanted. Argue away!
PS: my opinion (just an impression)... past scandals are irrelevant, players throwing tantrums or being rumored to do drugs is also irrelevant (NFL/NBA players involved in shootings, rapes, etc... mcenroe slamming his racket, woods getting caught cheating, lance getting accused of doping, etc.). The dirty barfly image is also not relevant - look at poker.
Inconsistent equipment and rules are a problem but not that relevant, we've settled on 9ft and the rules are all close enough. Nobody turned off the TV because the players weren't shooting it behind the line or using a bar box.
Lack of professionalism might be relevant, it gives potential sponsors a bad impression to see walmart clothes and empty seats when they watch e.g. a TAR broadcast.
Lack of business sense: probably relevant. Poker's been around a while but only got popular recently. Why? Probably smart people at the helm marketing it properly.
The one I've always felt was the main killer = it's dull to watch. Someone pointed out that poker should be dull to watch too, and that makes sense on paper. But I think people relate to poker because there are mind games and facial expressions to read, plus some drama in the unveiling of the cards. We like to see people trick and outsmart each other, even though that doesn't require any exciting physical acrobatics.
With pool, when the player is good, the rack is completely routine and predictable, and they make it look effortless. You don't say "holy shit" when someone drills in a long shot, the way you would when someone makes a 20 foot putt or 90 yard pass or something. Some of the stuff the athletes on TV do is out of our reach completely, but most players on this forum can make most shots you see in pro pool events... just not consistently.
