Good question. One I have pondered often myself. I think logically most bitcoins were lost when it was still a very fresh idea and people were mining whole bitcoins with a single so please pc setup. Because people never knew their value back then they were careless and didn't think it was a problem. Years later it is abundantly clear that this is a huge problem since the bitcoin system was no longer just some cool new technology but actually took off and flew like a rocket to over $1000 big ouch of you lost coins.