First, I'll just point out that your original comment talked about two separate things:
1. Why would one move to the US (presumably long term)?
2. Why would one come to study in the US?
The calculus is quite different between the two, because one can come just for the degree and leave.
Regarding 1:
It depends what degree you're hoping to earn in the US, and how many kids you plan to have ;-)
Pretty much anywhere in the US, with a SW engineering degree, you can ensure your kids have a decent education. And if you can't, you can simply move. SW folks are amongst the least tied geographically. Ditto for healthcare (at least in terms of affordability - quality may be great or middling, but usually not terrible).
But really, other than wages, the reason many choose to move to the US is language. They already know English. There aren't that many other options. The UK can be a really, really crappy place. I know many people who moved to the UK, spent anywhere from 5-20 years there, and moved out because of their social problems. They are all happier - including those who relocated to the US.
Beyond the UK, the main choices are Canada and Australia. Canada was the darling until recently. IIRC, a year or two ago it was ranked worst or second worst in terms of affordability amongst all OECD countries. As I mentioned in another thread, on a per capita basis, 50x more Canadians move to the US than vice versa. Every one of my Canadian friends who moved to the US said "Socially it was great, but kept having trouble finding jobs and paying bills when I had jobs". Some miss the healthcare, but most don't. Overall the education may be better there, but they all feel their kids are getting a better education in the US because they can afford to live in places with good schools. A few plan to move back when they retire.
(Don't get me wrong, if I had huge piles of cash, I'd move to BC in a heartbeat. But I don't have that money.)
Australia/New Zealand is the only real viable alternative. Most people I know who moved there 10+ years ago are happy. The recent ones are less so - and the worry is it's becoming Canada (unaffordable + low pay). Still, they like it enough that they don't consider moving to the US.
As to number 2 above, especially in the context of graduate degrees: It's simple. For most of the world, a US PhD is extremely valuable (compared to, say, one from Australia). So it's a no-brainer to understand why they'd come here for a PhD. Easy to find jobs elsewhere.