Hard to say easily. FWIW, I used to contribute to Wesnoth a lot (maintainer of one localization team) so I saw the dynamics and whatnot firsthand for about seven years. Freeciv is a fundamentally different type of game, so many of my points may not be applicable. Let's see...
- The game had both strong multiplayer and singleplayer (campaign) experiences.
- The game had a free multiplayer server with moderators, linked to forum, etc.
- A casual game did not last more than a 2-3 hours at worst, making it easy to stuff the gameplay into normal schedule - no big commitment. Campaigns could be saved any moment, again no time pressure.
- Modding (UMC) was made easy, identical to official stuff, with a dead simple format and a rather flat learning curve (WML, one object per image etc.).
- Mods were encouraged by an official add-on repository, accessible easily from the game itself
- The high graphics standard and excellent rule balance that you see over there has been achieved only after the community started expanding.
- The community existed from the beginning and was never seriously hampered (IIRC). Look back at how many times the Freeciv forum crashed, now GNA etc...
- The community was really welcoming for both players and contributors. /me points to cazfi's thread about holding back from contribution as a good thing
- The community was concentrated around a single project, with single code base, singular communication vehicles (forum, irc, wiki), single modding scene...
- The community had on a single forum spaces for casual gamer lounging, modding, and the core dev stuff, with no barrier except moderation. People who came to whine about overpowered elvish rangers could always see what the cool guys are doing, perhaps learn a bit about how it's done, and take part in the serious stuff just by posting about why does not their file parse instead of op ranger chat. Arguably, that's here too - or not, since the code "just happens".
- BDFL? Who is Dave's counterpart here? At that, who is responsible for this, that, etc.? We have neither a visible leadership, nor visible "doers", it seems.
I can't really point to a single thing. It's all and nothing at once. Really, building an community / player base around a project is something of an exponential process: When it exists, they will come and make it exist. At any rate, making best of what's here and now is better than dreaming up some glorious alternative!
On a final note, somewhere else I remarked that on the surface, Freeciv feels like a finished, polished monolith, inaccessible with no place to start making impact by changes. Arguably, Wesnoth has reached that stage too, and it shows. Six years ago, their forums bustled with activity. These days, it's a ghost town. Those who take a look only now just can't compare and are lulled by the numbers, but it used to be far more glorious. So in a sense Freeciv sems to have already "won", too