Because they're getting serious cold feet about if they can take Taiwan or not.
Philippines is far from an ideal candidate but an idea is that it's better to lose Philippines than to send the whole US navy into an attack on Chinese core lands, only to get everything sunk.
If China smashes Philippines into fine paste, US can at least argue for EU to oppose China economically.
The risks for US to bank on China to attack Philippines is much lower than for US themselves to come out at Taiwan, and can achieve the same economical effects regarding EU.
Another line of thinking is that China maybe will not give as much energy fighting an offensive war vs a defensive war.