Cost.
Raid 0 doesn't protect your data from a drive failure. Raid 1 requires you to have two disks for a single disk of space (or 4 for 2, etc, in Raid 10). Raid 5 is best of both worlds for something that doesn't require as high a speed of write. You can lose a drive and recover and only lose one disk out of your spindle worth of space. Also, depending on make/model, drive cache'ing and the like has come a long way to get RAID 5 much more competitive for speed.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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