True, Texas was its own country for 10 years. That's a big source of pride for a lot of Texans. Probably worth noting that they applied for Statehood with the USA almost immediately after winning their independence from Mexico. Internal politics of keeping a balance between Slave and Free States (Texas wanted in as a Slave state and there wasn't an extra Free State ready to add at that time) led the US government to tell them no. So Texans decided they'd do their own thing. They still joined up later on of course.
But all of that is
fftopic: anyways. The reason they left is simple: They were given the chance to be free for the first time in ages and took it. I don't think they trusted in the idea of a democratic USSR, simply because of the past history of the USSR. Threre was no indication that things would not suddenly change into the same USSR Communist police state the moment that Gorbachev or Yelsin were no longer in power. So they jumped at the chance to get off the boat. Who's to blame them?
We're talking about the former Soviet Union. Lithuania, Latvia, Estonia and Moldavia are excellent examples of countries that never had much desire to be a part of the USSR to begin with, nor the Russian Empire before it. Chechnia is a pretty clear example of that too, though it is still part of Russia today.
As for the Ukraine, it has its own unique ethnic background that is distinct from Russian ethnicity, so they might well feel that remaining as an independent country is a strong expression of that uniqueness. Also, as stated, they were conquered and not given a choice in the matter. The assassination attempt in the last elelction by the more pro-Russian party trying to kill the more pro-Western candidate, further deepens the divide.
Belarus' current government would probably welcome a reunification with Russia, but that may not be what its people want for themselves. Also, Russia is a very troubled country. Kazakstan, Uzbekistan, Turkmenistan, etc ... all are heavily Muslim and they're did their own thing long before Russia showed up and took over. They are likely to remain very friendly, but I doubt they'd want to rejoin Russia.
Azerbaijan, Armenia and Georgia all have their own disticnt past histories and no real reason to rejoin.
More than anything, what has a new USSR got to offer anybody? A marriage to Russia's economic nightmares and internal mess? The illusion that NATO or the USA is coming to conquer them doesn't work anymore. So banding together to face a common threat doesn't work. Have I missed something that would be a strong incentive to reunite though?