The fact that they had natural resources, knowledge, etc means nothing because, as Krasniy stated, they wasted it on weapons instead of things that people need. By the 1970s the Soviet economy was hugely outdated, and as the west moved towards computers the Soviets were still churning out steel and tanks in factories dating from the 1930s. Add the corruption of the Brezhnev years and there's a nice recipe for the collapse of an empire.
That's the economic side. The other part is that the USSR really shouldn't have come into being in the first place. A good chunk of the various ethnic groups which made up the USSR were forced in, and the only way they were kept in and quiet was by force as well.
Now enter Gorbachev. Gorbachev, initially, wants to reform the system which is already in place. He wants to change the USSR, make it more open, and efficient. By 1989 he finds out that he can't change the current system, so he has to take it down completely.
While he's letting people speak their minds, they come to the conclusion that the USSR sucks and they resent its domination over their nations. This brings us events like the anti-Russian riots in Kazakhstan in 1986, Baltic independence movements and other nationalist movements springing up everywhere. People now start to like nationalism more than communism.
Gorbachev, meanwhile, is swinging back and forth between hardline and reform elements of his government. In the winter of 1990 he allies with hardliners and starts putting them in his government - Valentin Pavlov as prime minister, Vladimir Kryuchkov as KGB chief, Yanayev as VP, etc. These guys have their own agenda. Gorbachev doesn't see it and proceeds to re-negotiate the Union treaty with his counterparts in the SSRs.
Now I'm sure we all know about what happened August 19-22, 1991 so I don't need to go over it. When it's over, a) Yeltsin is a hero and Gorbachev is now powerless, b) people realize that as long as the USSR exists, people like Yanayev and co. will always exist to try and turn back the clock. Yeltsin gets the upper hand and pushes Gorbachev out of the way. In quick succession the SSRs declare independence and it's all over on January 1, 1992.
Why'd it fall? People realized that the system sucked. They saw economic near-collapse and the coup in August 1991, and they concluded that as long as the USSR existed it could always go back to the old ways. Gorbachev didn't move fast enough to take the system apart, and put his trust in the wrong people. National movements gained steam, and then it was all over.
Wow. This is about my longest post ever.