I think the number one reason, hands down, people don't even bother to play one game is the graphics. Why play something so ancient looking when there's things like TF2 and Crysis on the market? There's no simple solution to this problem however, but in the meantime, we can still utilize better looking textures (as y00tz is helping the community do so) and spend more time in the aesthetics department during map development. No blitz, sankyou.
(On another note, the menu for Paintball in-game is pretty ugly. Someone should step up to redesign it!)
Past that, after a few games, players tend to realize the elitist community. The word "n00b" seems to fly around every 30 seconds on the public servers. If someone asks for help, they are left in the dark and the game continues. Not to mention the rudeness and racism you sometimes see on the servers that probably annoy new players. The articles on DP Resource, such as on jumping and the like, mitigate a problem a bit, but it requires players to know about the site and then practice on their own time. Then again, who wants to practice when they can just play other games? Newbies probably feel very inferior (and useless) in maps like airtime3.
Beyond that, if you suck, you suck. There's nothing that can be done about it, but I suppose that's a problem with the gameplay. In other games, i.e. Enemy Territory or TF2, there are alternate things you can do if you suck so much at getting players eliminated. In Paintball, if you suck, there's not much else you can do to contribute to the match. (Though arguably, this leads to newbies camping and lining, which also invokes the elitist attitude of other players, and even then, camping and lining are not 100% effective.) How this can be solved, I am not sure.
New players, besides being fed up with the above, likely get bored of the same maps [that they suck at] being played on repeat. The map gameplay never changes. However, before build 20, adding maps to be downloaded seem to have been a suicidal move because no one wanted to wait several minutes to get a map. The larger maps take up over 10 minutes to download, so you've just wasted probably half of the game obtaining the map. Build 20 should fix this though.
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For a while, I was testing what settings on my server would make people actually want to use it. I didn't try to target veterans because the ping is so bad, but newbies don't know any better. Using maps that required any downloading left the server empty -- although people would connect, they'd drop out of the download ASAP. When I used maps that emphasized jumping like ub_rooftop or airtime3, the server was dead empty. When I fell back to maps with very little jumping, i.e. ub_cliff, there were players on the server. Also, to note, ub_cliff has "alternate gameplay" -- lining and camping. (These experiments are the basis for my statements above.)
Also, on another observation, once I accidentally made guntemp_dec 0 so your gun would stop shooting after one shot. I didn't realize this for a few days, but oddly, there were a number of newbies on the server actually enjoying themselves when I decided to come on.
