Originally Posted by
Talanic
If I remember right, it was because humans could multi-class. Other races could dual-class, having two classes at a time, but humans could play as one class, go for a while, then say "Nah, done with this one" and start leveling in the second class. This was only applicable if you had pretty good attributes - like a 14 minimum for primary attributes for your new class.
When they did this, they lost all benefits from the first class except HP (which was a big thing because you only gained hit dice up to level 10 and then only gained one hp per level if I remember right). They only regained their original class abilities when you exceeded your previous class's highest level. Then you regain everything, but still have limits because certain class features are locked when you're wearing the wrong armor types. Oh, and you could never go back - once you multi-classed, you were done with the previous one.
You can see this being made into pure cheese in some of the Forgotten Realms books. Elminster in particular was a fighter first (for the health and attack bonuses), then a thief, then a cleric, and only then a wizard.