Because, like it or not, the US remains a significant trade partner for the ASEAN region. The US is also a vital security partner, in the sense that it serves as a powerful counterweight to China who, if it had more militaristic designs on the region, could give ASEAN a very hard time.

Naturally, the Chinese would argue strongly against this view since they are of the view that a more appropriate regional security architecture would have China playing a more central role. But that would be a tough sell in the region since the Chinese government is still nominally communist, which makes it problematic for other governments in the region to "sell" more extensive partnerships with China, given the decades of anti-Communist indoctrination they have subjected their populations to.