i think the premise that southeast asia is seen as more connected to china than india, explicitly, is kind of wrong; it's more that southeast asia is seen as existing within the east asian sphere (of which the dominant powers are china and the united states), rather than the south asian sphere (where india is really the only significant country at all).

the reason is actually connected to the way the united states carved the world up after world war two, which itself was a consequence of the fighting during the war - this region was broadly occupied by japanese forces, and then came under american command after the war.

but, there were of course also conflicts here during the cold war, which led to chinese-backed and soviet-backed client states. india, on the other hand, was broadly non-aligned.

it's within this entire context of this conflict theatre that the association comes out both in terms of economic reality and in terms of academic categorization.