What would prevent a man from filing for a divorce especially after it has been years since initial separation and he has been in a serious relationship for a year with someone else?
There's the "losing half your stuff" part.
The potentially having to pay spousal support part (depending on each state's laws), which he may not have to do during an unofficial separation.
Then there's just the legal minefield of divorce, and since he has moved on in every other way, maybe he is just not wanting to deal with that one. (My actual divorce took only a year total... but the larger process took 6 and a half years, and a big part of it was this - even though I was the one who asked for the divorce, there was just this big "ugh, don't want to deal with the government!" ominous feeling).
And then there's the stigma, real or imagined: you failed at one of the biggest things there is in life. The actual divorce paperwork is an evidence and proclamation of that failure. Subconsciously, maybe he isn't ready to accept that, even if at the conscious level he has moved on to new love and relationship.