In just about every edition of D&D (save for 4e), Paladins are required to be Lawful Good. If they stray from that, they are completely stripped of their powers. This means that evil deities can't grant powers to paladins or if they do, they go into a new class (e.g. Anti-Paladin).
My question is: why? Why was it designed that paladins have to have such a strict alignment? It seems to me that it unnecessarily pigeonholes the character types and doesn't make sense in D&D world. After all, couldn't evil deities have holy warriors?
I also don't understand the mechanical decision about why was it designed that an evil Paladin has to be a different class. Wouldn't this create a problem if you wanted to redeem an evil Paladin into a good one? This never really made sense to me until 4e where they just dropped the Lawful Good restriction entirely and let you have a Paladin of Vecna (or have that Paladin of Vecna become a redeemed Paladin of Pelor without having to switch classes).
I'm especially interested if the D&D designers ever wrote anything about their decision to make it this way.
EDIT: I'm mostly looking for an answer on why the rules were designed this way, not reasons for why non-Lawful Good paladins can't exist in the rules as they have been written by the designers.