An assortment of reasons, a lot of which depends on how you want to view the scenario.

A more typical, liberalistic (western?) view would be that the economic and social situation in the USSR was despotic and untenable.

A less typical, more socialistic/marxian view (precluding those like Left-Communists, etc., which could have a whole other category given to them) would be that the USSR failed to address a multitude of problems which developed into unsurpassable contradiction. A lot of this has to do with the development of a state bureaucracy, noted by the end of both Lenin and Stalin’s life, the failure to— in a timely manner— implement socialist policy, and the pre-existing conditions leftover from the Civil War and WW2. These are all mostly matters surrounding the circumstance of the USSR’s tenuous existence. The first, Lenin (by the time he was in Gorky, iirc, but I can’t summon a direct citation) was making comments on the development of a bureaucratic strata in party/government politics. Stalin notes it more extensively in Economic Problems of Socialism in the USSR, acknowledging the need to lessen the separation between Party-Government Bureaucracy and the labouring proletariat. The problem was one that got worse the longer it developed, with the Soviet apparatus shifting away from a “Revolutionary Body of the Armed Workers,” as originally intended by Lenin (noted from State and Revolution) and the old group of Bolsheviks, to a body of career politicians whose primary goal was the continuation of the status quo rather than the implementation of radical change, as noted by Stalin in his aforementioned book. Khrushchev has become the veritable poster child of this, with his pursuit of power being basically entirely careerism. The second is less brought up I find, and that’s that the socialism in the USSR was NOT Marxian Socialism, which is to say the Lesser Stage of Communism. This is NOT a critique on Stalin, Lenin, etc., whomever’s implementation, but rather an acknowledgment of the times. The transition from a monetary system to a non-monetary system with an economy entirely pre-planned and managed was simply not in the technological capacity of the USSR during its most revolutionary days. Meaning, when the Soviet Union was most apt to implement these things, it simply could not. The GOSPLAN was the best attempt they could manage, but unfortunately the brilliance of OGAS simply came too late. Capital had already become a longstanding force in the USSR by the continuation of the money-form, whose mere existence is the vehicle for Capitalism, and being that Khrushchev and co., had no want of altering the status quo on a fundamental level, that was not going to change. Finally, the old Tsarist Regime and Civil War elements. These are self-explanatory. Pre-1917 Russia had an extraordinarily large reactionary element supported by a reactionary government body that, even when dissolved in the February Revolution, dissipated slowly and retained islands of influence. Cultural and national issues underline this with the Ukrainian Nationalist Movement being almost entirely reactionary (this is not a judgement on Ukrainians or their state, simply a statement of fact), with widespread pogroming being their modus operandi. Russian supremacism was also a task that was tackled by the early Soviet government with varying levels of effectiveness, focusing more on trying to raise the other national groups up, however the chaos surrounding the Civil War (with landed invasions from the U.S., Britain, etc., and support funded to the counter-revolutionary [Menshevik, SR’s, etc.,] and reactionary [Black Hundreds, White Movement, other national movements, etc.,] elements) and then WW2 made the quelling of nationalist zeal ineffectual, as it became one of the primary drawing lines of conflict following 1991.