Heres the WHOLE STORY of why Scott left.
Blink 182 never started out with their current member Travis Barker but with their original drummer Scott Raynor. Contrary to popular belief, Tom DeLonge and Scott started the band and then Mark Hoppus joined, quit, and then re-joined. While the band's success was growing and Blink 182 was on tour, arguments were forming between the members.
Scott wanted to take the band in a different direction than Mark and Tom did, so all was not well within the trio. Scott started disappearing between shows and even skipped out on a few so Travis Barker of the Aquabats was asked to fill in a few times.
Scott Raynor kept pulling further from his band mates and at the same time he was drinking excessively. He was told to clean up his act and he asked for the weekend to think about it. Scott decided to quit drinking, but it seemed that it was too late and he was already fired. The Blink 182 line-up was broken and Travis was hired permanently.
The whole situation seems unfair, but that's pretty much the way it went down according to Scott's Absolute Punk interview. Mark and Tom remain silent on the subject so not many people know for sure what happened when the original Blink broke up. Scott is currently in another band and is still working with the organization Stand Up For Kids. Listen to Blink's song "Man Overboard"; it seems to explain the story pretty well.
From:
http://blink182.punk-online.net/info/scottstory.html