Neville, why would you say only non-MO follows the psak? In general, MO do not use eruvin in Brooklyn or Manhattan. R’ Schachter shlita paskens not to use it, with only one exception that I know of (which I’ll get back to).
The ones who don’t follow R’ Moshe on this issue are mostly chassidim. They’re the ones who are behind this new attempt, and also backed some of the earlier attempts.
As for the exception R’ Schachter gave, I only know of one. By Mt. Sinai hospital, there is an eruv around one side of it, allowing people to walk from one building to another. One street is bordered by Central Park, and there is a dead-end a bit further down on the main street the hospital is on, I believe.
I spoke to a friend who was a student of R’ Schachter’s about it, and asked him why he’d allow it, when R’ Moshe was opposed. He told me he’d asked R’ Schachter that himself. The response was, “People are dealing with a difficult enough situation when a loved one is in the hospital, especially over Shabbos. So I looked to see if there was a way to be lenient in this situation. In the end, I found that the situation around the hospital is actually not the same as the rest of the areas R’ Moshe paskened about, and I felt that R’ Moshe himself may have been lenient in this case too.”
My friend also told me that he heard from another close student that R’ Schachter left out one piece when he responded. He was so scared to go against a psak of R’ Moshe, even when he felt that he wasn’t really contradicting it, that he fasted before issuing the psak.