I suppose each of these reasons plays its part, and the crucial ones will vary from gamer to gamer according to taste. My own theory is that what puts off a lot of potential players is simply the perception that it was a one-sided walkover: a short war in which the French got crushed and won hardly any significant battles.
This set me to pondering: how much (or little) appeal is there in wargaming one-sided wars?
The most popular wargaming periods by a mile are WWII and the Napoleonic Wars. Of course these encompass a large number of individual conflicts, but still, you could make the argument that part of the interest is that they were titanic struggles that could have gone the other way, in terms of both the overall result and many of their important battles and campaigns. Hence when we wargame these, we can feel that we could have changed history. In the survey I'm citing, from Wargames Illustrated, the one discrete conflict to register is the American Civil War, and the same argument could work there too. (Though that argument doesn't seem enough to make WWI nearly as popular.)
In the case of the Franco-Prussian War, arithmetic plus hindsight makes it much harder to believe that France could have avoided defeat. Thus all we can hope to achieve as the French player is to do less badly and lose less quickly and disastrously. There are plenty of players, myself included, for whom that is fine and we are happy to play on those terms; but there are also plenty from whom the cry will go up, "But I couldn't have won anyway!"
That cry is an actual verbatim quote (RIP, Phil), not from an FPW game, but from a game set in another one-sided conflict: the German invasion of Poland in 1939. Yet my impression - not founded on any survey statistics, only on casual observation of show games, manufacturers' output etc - is that gamers are keener to play 1939 Poles than 1870 Frenchmen, even though they were surely even more certainly doomed to defeat. Is it that a gallant Polish army giving Germans a bloody nose in a David-and-Goliath fight is more glamorous than the fumbling ineptitude of Napoleon III's Second Empire?
There must be many other examples of one-sided wars: the Russo-Finnish Winter War, the Greco-Turkish War of 1897 (both of which have given me some very good games), various colonial conflicts such as the Anglo-Zanzibar War, and no doubt lots more that I'm unfamiliar with because they pre-date my preferred C19 and C20 periods. I'd be interested to know which one-sided wars readers of this blog either love for their glorious underdogs, or avoid for their futile inevitability.