Looking at the old DCI Tournament Rules, circa Ice Age in 1995, we stumbled across a funny passage:
"After reading the updated rules carefully, you'll note that Summon Legend cards are no longer Restricted across the board. Yes, that means that you may now construct a sanctioned tournament deck with four General Jarkeld cards in your deck! That doesn't mean, however, that both players in a given duel may each have a General Jarkeld in play; the rule pertaining to only one individual legend card title in play at any one time is still effective!"
No, it was a combination of two things. One was for rules reasons: if you only had one Tor Wauki in your deck, you wouldn't be tempted to have more than one in play very often. Kept things simple.
The other reason was for flavor. These cards represented powerful Legendary beings, and you only got one chance to summon them! Make it count!
I think we're all glad the DCI stopped dabbling in flavor and started restricting cards based only on power level concerns.