i honestly think, after struggling through 2002 and seeing the way that Venus and Serena seemed to have all grandslam finals wrapped up for the rest of the decade (in 2002, it really did seem to be the most likely outcome), she probably thought, whats the point. then in 2003, with Venus and Serena still dominating when they played and with the Belgians dominating also, she probably felt the same. I think as 2004 progressed and into 2005, it made her think, actually, i could give this a go, and she did. well done to her.
I still think Hingis would rather take on 7 top russians to win a grandslam than take on 4 nobodies then have to go through, Serena then Lindsay then Venus.