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.
The first thing I did after buying BTC/LTC was to transfer these onto external hard drives to ensure these are offline. The wallets are encrypted (60 digit passwords) and these passwords are stored safely on USB flash drives (document with passwords is also password protected [memorable long phrase]). Nobody is going to step in and help you when it all goes wrong therefore ensure you have taken every step possible. I limit my exposure to hackers by leaving as little as possible on these unregulated exchanges and keeping my wallets offline (I hate the idea of these parasites getting my money. They are scumbags, but they are not going away anytime soon). If I offered to store your Bitcoins would you accept this? If yes, I would be happy to do so.
It's never been my intention to offend anyone, however people complain on forums about losing money, however they never took reasonable attempts to mitigate the risks. It gives Bitcoin a bad reputation that it doesn't warrant. Banks willingness to refund people has given a false sense of security around money (Bitcoin is unregulated therefore there is no help when it goes wrong).