I think most gox's users have left their bitcoins on the platform for two reasons:
-To buy and sell bitcoins quickly, without transferring bitcoins on their Mtgox account.
-They thought Mtgox as historical platform was the safest place to keep their bitcoin.
You can not blame them for that ...
Leaving large sums of BTC/Fiat on unregulated exchanges is reckless. There were enough posts on this forum warning about this exchange (there are no excuses). BTC was created to be decentralised (wallet).