I have a hunch that in the USA at least, it might be because Filipino food has to compete with the cultures that influenced it. Chinese and Thai food are very popular Asian foods here and Filipino noodle dishes need to compete with them. Latin American food brings a lot of Spanish flavors that Filipino food also has to compete with.

If an American thinks "spring rolls and noodles" they go to their local Chinese take out, not a Filipino restaurant. If they're thinking about empanadas, they don't think Filipino, they think Mexican or Colombian.

Filipino food might have an identity crisis here that stems from a lot of its culinary identity being from Chinese and Spanish influences. This in turn might make it hard for restaurants to find their niche outside of the Philippines. Insofar as restaurant businesses generally build customer bases by establishing some kind of identity, this might just be too much for most Filipino restaurants to overcome in a hypercompetitive industry. Glad to see some entrepreneurs try though.