Another possibility is that many people have never heard of these arts before. Everyone has heard of Kung Fu and Karate, and TKD is everywhere these days, but what is the chance that a random someone who is interested in taking "a martial art" is going to know to look up Kali or Escrima?.
A related point is the
"Cool-ness" factor. In the 60's when Karate broke out of obscurity, it got used in the movies and media. Elvis did it. James Bond and all the other cold war super-spies did it. And there were the "Easterns", Japan's re-interpretation of Americas "Western" movies. Films like the Seven Samurai, Yojimbo, Sanjuro... Basically, Karate became Cool.
In the 70's, Bruce Lee's movies and the "Kung-fu" TV series brought CMA into the limelight. The 80's brought in Ninjutsu. And, finally, Hollywood's excess as well as the McDojo phenomenon began to make it all look like a joke. MMA stuff is now all the rage. And when you consider how cheesy the current presentation of "traditional" martial arts has become, is anyone surprised. At least UFC stuff is
believable.
If FMA ever got a notch above obscurity, it was probably due Dan Inosanto. But outside the martial crowd and Bruce Lee fans, I don't think may people even noticed. And what attention there was seems to have been focused more on JKD than FMA. Somehow the FMA remain unknown at large, and
the Philippines as a culture has never gained the coolness factor associated with Japan and China. How many Anime cartoons are set in the ancient Japan or China? How about the ancient Philippines?!? And video games... Any body ever hear of anybody ever play
"Archipelago of Mayem" ... the one based on tribal war in the Philippines? Maybe because it doesn't exist! ...yet. Or, how about the classic,
"Grand Theft Datu"?