[Steko]

This is an oversimplification, most Americans will also be quick to categorize darker skinned Southeast Asians as Asian.

Historically most Asians in the US were from East Asia and later SEA so the label tends to evoke those groups. Lacking familiarity, it may also be harder for many Americans to easily discriminate (lol?) between East Indians and other non-white/non-Asian groups.

Still there are plenty of people that do include Indians as "Asian" in the US, it's hardly a universal standard to exclude them as a lot of comments here seem to think.