I have always assumed it was to reduce the likelihood of conflicts with future alarms. For example if I set alarms at 8:00 and 9:00, if I've been snoozing the 8:00 alarm, and snooze is 10 minutes, the two alarms will both fire at 9:00. With 8 or 9 minute intervals that is much less likely, and so its easier to see that maybe they are two different reminders. This probably applies more to phones than old school alarm clocks however.