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).
Come on! for years mtgox was the least worst option. It might surprise you but a lot of people new to bitcoin don't know how to create a wallet an move the btc. It does not surprise me that mtgox had so many btc left on the exchange.