There was also the little fact that Kasparov was not playing his top chess in the 1997 match - the loss in the sixth game was considered very bizarre at the time (him going for the Caro-Kann, while not having much experience in that opening)

His rematch against Deep Junior (the 2003 computer) ended in a 3-3 tie, so humans were actually able to hold off computer for quite a bit longer than 1997...