Because of many factors. The privatization process would give vouchers for shares of state owned factories etc
But to get a major part of vouchers you had to be someone or know somebody. The old communist elite was naturally the closest to the process and settled in. It was a free for all and yeah, west was too busy celebrating ussr falling apart to notice regular people getting fukked over. And regular people had no concept of economy or basiccapitalist market so they fell prey to every ponzi scheme and pyramid in existence.
One of the main reasons Putin has this power because most Russians hate the 90s when they were dirt poor there was chaos in the streets and everyone was corrupt on every level and there was no one they could complain to.
Part of soviet union nostalgia was that even tho people werent rich there was stability and order, even if the party was corrupt.