The short version: the show is good and sincere (but trying it in the first place requires high openness), a large subset of LW's current readers are into fanfiction, and the brony community in general creates lots of fan materials.

The 4th generation of the show started 2 years ago, and is very well-made, is happy, and less targeted at little girls than the previous versions. Generally, boys shows got more talent and funding because network execs knew that both boys and girls would watch boy shows but only girls would watch girl shows- which was, in part, a self-fulfilling prophecy. Why watch technically inferior shows? (Most male fans of the current generation find the design of previous generations hideous; I don't know how the female fans feel about it.) The show still has a definite feminine feel- the protagonists are all female, and the majority of the episodes center around personal interactions and friendship problems- but there are also "save the world" adventure episodes, and a strong diversity among the characters in terms of both skill and personality. The personalities are iconic and basic enough that most fans identify strongly with at least one of the characters.

I only gave the show a try, though, when I needed to watch television and had watched every other show on Netflix made in the last five years that I expected to enjoy. (I was only recommended it once or twice.) Hearing that a show for little girls is actually good and giving that hypothesis enough credence to watch the show seems like something that requires high openness- which is a trait I suspect is associated with being an LWer. (When much of your social circle approves of it, the amount of openness required is obviously lower.)

One of the things about the show that's polarizing is that it's very sincere. Ponies are, in general, happy and like each other. Compare to, say, Watchmen or The Wire. One way to show sophistication in modern society is by appreciation of irony, but LWers strke me as more likely to construct their identities out of sincere enjoyment rather than ironic enjoyment. The sentiment that, really, 2+2=4 and, really, capitalism creates wealth seem to resonate here much more than they do in some other places. Somewhat related, it seems possible that people who are less socially adept are more likely to become bronies, but I don't have enough details there to more than speculate.

As for writing fanfictions, I found LW through HPMoR; when I heard "here's some Harry Potter fanfiction, and it's really good" my response was "okay, I'll give it a read" not "fanfiction? And for a children's book?" Similarly, once I started watching MLP I started consuming a lot of MLP fanfiction- and so after a while the thought of "hey, why not write my own fanfiction?" popped up more and more frequently until I put pen to paper.