I was chatting with a friend the other day about the whole Bernie Sanders vs. Hillary Clinton thing, and it occurred to me that there might be a pattern of independent office holding in the US, especially in Congress. I had the impression that the Northeast and Minnesota, together, had the lions share of such individuals. It turns out I was partly right and partly wrong.
Going back through the 1970s, and for the heck of it, including Governors, there have been 19 such individuals, and they are indeed concentrated in the Northeast (with a couple in Minnesota, as well as Alaska, Virginia, and Florida). Here's a map:
If you go back over the history of the US, there are a LOT in the south, and the pattern is overall quite different, but the time periods are not comparable. The current structure of the major political parties in the US, and thus by extension, of independents, came to fruition by, and not long before, the 1970s.
Why is this interesting? Well, it may not be. But Bernie Sanders is one of those office holders, so this could be important context. The fact that he represents Vermont, which is in the part of the country where this sort of thing is common, makes sense.
I think you have to differentiate a bit between statewide and local Congresspeople, and note that for the most part the states with independents seem to be small (population wise).
Note that a big chunk are in relatively cheap media markets, and in New York, Florida and Virginia, you have fewer officeholders and they are in smaller districts.
In Virginia Harry Byrd also had a stint as a Democrat, and he's the only statewide guy. In New York there hasn't been an Independent governor or Senator (I went back a century). In Florida you had Charlie Crist, who left his party, and Lieberman in Connecticut who did the same.
My sense is that if you want to get in as an independent you need money and that comes from the parties a lot of the time. That means winning statewide office in New York is going to be really, really hard if not impossible.
Contrast this to Minnesota, which is one of the cheaper media markets in the country. (Just ask the Minnesota Twins :-) ). Retail politics is just easier to do there, as it is in Vermont and Connecticut.
This is why you'll see more independents as local Congresspeople and (Note your map) in areas that have cheaper media markets.
By the way I was curious who you had from New York, as our current delegation is 18 Democrats and 9 Republicans. We've had some reps who were third-party people, though they often ended up as Republicans or Democrats eventually (we have a fusion system here).