I think that the Genre of FMA is very complete when it comes to combative application, first of all. So people who start with FMA can get just about all they need w/o having to go outside of the genre, although it still happends often.
Also, FMA has a niche when it comes to weapons training, so most people who study different arts that don't focus as much on weapons usually decide at some point in their training that they need more weapons work, which brings them to FMA. Also, FMA makes a good supplement art due to the Filipino culture, and the way the art is taught.
Also, in terms of practicality, I think FMA lies on the top of the chart. The reason is because most FMA haven't been that far removed from actual combat. The Philippines, as beautiful as the country is, is still 3rd world and war torn. They have been through constant military struggles, regime changes, dictatorships, and chaotic rule within the last 100 years. Most of the old masters and grandmasters either trained under people who had to use their skills in real combat, and often they had to use their skills in combat themselves. So many of the FMA techniques and skills have been "road tested" very recently. Since the FMA haven't been far removed from real combat, it just makes for a very practical genre of martial arts. Due to this reason, I often find many people who study a different art as their "life study," but they learn FMA to get a practical sense of "how to fight".
So, I think these may be a few reasons why people seem to "end up" at FMA.

PAUL