We could probably inexpertly divide this into to reasons or camps, though the reasoning inevitably overlaps. One reason is simply practical - getting things done officially was often slow, ineffective, 'unfair' (as perceived by the recipient so maybe it was or wasn't actually fair) and so on. So why not do something that bypasses bureaucracy, rules etc. in order to get a better outcome? The degree to which unofficial economic exchanges (from bribery to other exchanges of favours, calling in favours etc.) supplemented an unofficial system may have even been necessary for the system to function. Kotkin argues this as a system wide/industrial fact in "Magnetic Mountain" and you could probably read a more consumerist interpretation in Natalya Chernyashova's "Soviet Consumer Culture in the Brezhnev Era". For more every day perceptions or examples of bribery and similar you could have a look at Sheila Fitzpatrick's "Everyday Stalinism" (eg. unofficial channels to get a better apartment) or Donald Raleigh's "Soviet Baby Boomers" (for example using pull to get your children into a better university).

Another argument would be that this was somehow cultural or otherwise ingrained into society. J. Arch Getty argues this in "Practicing Stalinism" - that in effect bribery, use of patronage networks to get what you want etc. in large part reflects a perception that real power in personal, and that official agencies are just fronts for powerful individuals or unfundamentally useless. So in this argument bribery wouldn't just be a question of it being a practical way of getting things done, but that it was just assumed to be the normal and real way of getting things done.