The most plausible explanation - in my opinion - was that their opponents were able to use the longer thinking time to avoid the tactical traps (knight forks, skewers, hanging pawns, etc.) and put the game onto a more strategic level where the blitz players may have been weaker or at least less experienced.
One kid, he was 15 and I 21, had impressed me as a really tough player in blitz games; always finding a sharp tactic instantly, every move played confidently and always threatening something. He'd win at least 2 of 3 blitz games, and probably 3 of 4 lightning (1 minute) games against. However, at game/25 minutes, it was just the opposite. Quite a few of the other fast players were like that too. Not all of them though, many of the best "slow" game players were damn good blitz gamers - at least at the clubs in SF, LA, and NY that I played.
But I make no claim that this is usually the case! Still, as has been written about in players' bios and articles, Blitz can be an art itself and Blitz specialists - especially if they make some $$ that way - can become quite deadly.