What were some of the reasons that hindered reforms in the USSR?

How long have you got? Roughly (1) the system was designed to stop people making changes, so it was very difficult to get people to see things needed to change (2) lots of people within the system were doing very nicely out of it - the Communist party boss in Uzbekistan is said to have made up to 1 million roubles per day (per day) in bribes in the 1970s and didn't want to change it (3) the system itself was based on a very particular ideology, known as 'Leninism,' and any changes to it might threaten the whole state, so people didn't want to rock the boat (4) the USSR was a very poor country which was at the same time spending huge sums on armaments, so there was never enough money to go round to pay for changes anyway. There were others, including being largely cut off from the outside world, a reluctance to speak openly due to the legacy of terror and censorship, and a shrinking population, but those were the main reasons the system wasn't very easy to change.