The Soviet people voted overwhelmingly in favor of retaining the Soviet Union, albiet with reforms, in a referendum that was ignored when the leaders of the USSR’s constituent republics agreed behind closed doors to dissolve the nation.

The referendum (the only one they ever had) with it being in 1991 it was already a much different Soviet Union than we usually think and very late in its life as an effort to somehow keep it together, even though in a pretty different form. The wording makes it so that there was very little reason to oppose it unless you were a hardline independence advocate (so you might not respect their authority anyway or don't want to give them credibility etc) since independence or no, it was promising more independence, human rights, freedom and so on. And in some countries that was tied to "let's become independent at the same time but also keep in this new federation or what have you". So it wasn't even a "should we keep Soviet Union or not" but rather "should we make the union different, better", which again, not much reason to oppose it no matter what you thought. Keeping it as it had been was the hardliner approach of keeping the older style Soviet Union and that wasn't very popular.

And the new treaty was never signed because communist hardliners tried a coup to reverse the course. The attempt backfired horribly and just lead to even swifter dissolution. But I'd say it was already heading towards that anyway with people seeking to break away from Moscow and the whole system in a turmoil over reforms (to some too radical and to some not radical enough). In hindsight it feels like they would've needed a miracle to keep it together in any recognizable form.