Why was Kird Ape banned in very first Extended ban list?
The simple answer is that at the time of the banning, WotC saw this card as a problem for the metagame in Extended. It could put an opponent pretty far behind pretty early, and many decks had to tailor early plays just to deal with Kird Ape. As Mark Rosewater notes in an article from 2003 (emphasis mine):


  We ban and restrict cards because we believe there is something worse than not allowing players to use a particular card, and that is having a play environment become so degenerate that the game is no longer fun. Sometimes individual cards have to be sacrificed for the needs of the larger game.


When an individual card takes over a format because it seems unbalanced, WotC bans it in order to try and change the metagame. This is constant battle for them in the Modern format, where they're trying to figure out a way to ensure it's a four-turn format.



If it's any consolation, in the same article by Rosewater he admits the Kird Ape ban may have been a mistake:


  Why did you ban Kird Ape in the original Extended format when Hypnotic Specter was not? 
  
  I’ll let you in on a little secret. We’re not infallible. We occasionally, gasp, make mistakes. We correct them when we catch them, but hey nobody’s perfect. Was the above a mistake? To quote my grandfather: “If it looks like a duck, quacks like a duck, and has a giant sign on it that says: ‘I’m a freakin’ duck!’, odds are, it might be a duck.”




I'm also skeptical that you are going to find a direct quote online referencing the Kird Ape ban. The earliest articles on the WotC website for Magic are from December 2001. I legitimately can't figure out when Kird Ape was actually banned and then unbanned, but this timeline shows Hypnotic Specter being banned in October of 1997. Since the above Rosewater quote references the banning of Kird Ape instead of Hypnotic Specter, I can only assume Kird Ape had both its banning and unbanning before then, more than four years before WotC decided to start using the internet to disseminate information about their game.