I think attachment sizes were also limited because back in the day, you know when Internet connectivity was a series of beeps and clicks, people used to surf the internet on these shitty little things called modems that ran at 33.6 and 56 kbps. Forget the 10 meg limits imposed in those days, it would probably have taken an hour to download that single message!

A lot of the hang up may have remained from that.

I remember the days when it would take 20 minutes to download an mp3 only for it to drop out in the final minute. Grrr!

Add to that what lloeki said, it's probably one of the most important reason. I becomes very easy to take down several servers at the same time!