I'm sure IW would be a lovely place to host the US Open, and they could expand to accommodate. I actually won't be surprised if it eventually happens. But do you see my point about it being a luxury venue? Not so many "regular Joe's" would get to go. Sure, there's "big city BS" in New York, but isn't there in London and Paris? (I suspect, but don't know, that Melbourne is a little more laid-back and easier to deal with, and perhaps even more affordable, aside from the airfare.) Every Major has its locals, and those who come in especially. Whether you come from close or far, for those who make the trip for the tennis, it's nice to have other things on offer. It's a destination. Indian Wells is not what most people would consider a "destination." Sure, you could visit LA and Joshua Tree while you're there, but not at the same time. Aside from the tennis tourists, and I think this is important, there would not be that many locals who could come. It's too far from anywhere, and the average ticket price is much higher than the US Open in NYC.
As I said, I've never been there, so maybe
@Jelenafan can say more about what it's like, having been there this year.