DIGGA, good question. I believe the low player count is a result of several key factors:
1. Though some people (including me) loved the original Fallout games, including its AD 1998 graphics, many do not. Most "modern fallout fans" only like modern graphics styles, and when they see this game, they actually say "ew" at the graphics. Nevermind the fact that Minecraft often looks worse with its 16-bit blocky snorefest, and that World of Warcraft's graphics look like a blurred cartoon, those are favored. I never understood this but that's the way it is.
2. FOnline games have no budget, no funding, no profits, and thus no scheduled staff. They run on donations only, and this will likely never improve. Thus advertising is also nearly nothing. Same for immediate help from an online GM. A lot of gamers don't like that; they get the idea the game is ignored by its own administrators, and they leave.
3. In the past 7+ years, many of the gamers who would play this game, already tried, and quit. (Why this is, is another good question.)
4. Despite the fact that any player here can download the developer tools, draw maps, build dialogue files, write quests and contribute playable content, the game severely lacks PvE content. There is no main quest. No GECK. No cut scenes or talking heads, and nothing like a template of trials. We do have some quests, and they are excellent and some are repeatable, but not nearly enough. We need more of this. (You're invited to help.) The game is wide open, and we players like to say "you write your own story as you go". There are numerous things to do, but it's wide open with no set route.
5. The FOnline community was already splitting up before 2238 closed down, but the end of 2238 pretty much fragmented what remained of any unified player base. With more servers, but not more players, we end up with fewer people per server.
6. This game community is mostly all jackasses who don't play the game for its explorable content so much, but rather use the game as a place to express their hatred. Not all, but most. The other 14 of us (lol) try to help people so they don't quit early, but the jackasses rule, and thus new people are run off.
One of the favorite things the jackasses here like to do is send infiltrators into your faction, posing as recruits. They destroy newcomers' efforts to get established, which often leads to new players leaving. There are, after all, so many games out there to play - why play one where you're stomped into the ground faster than you climb the learning curve? It's why I added a Guides category to the wiki, and spend hours on that.
So it's not a game for anyone, i guess. The harshness only appeals to those of us who like the graphics, and enjoy a tougher hostility challenge.
PS: Just kidding about only 14 good people here. I'm sure there's more than 14. I could be wrong about that. I think i saw 1 last month.
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All of that said, i say this is the best game i ever found. It's my favorite. Over time, devs have done amazing work to improve it. For details, try to read the long history of change logs, starting with 2238 and going thru Reloaded. I love this game.