I suspect Bill is the type of person to focus intensely on one thing at a time. Prior to 1997 his focus was Microsoft. After that, he rode its success into his next endeavor, which is charity.
You should read this article from 1996 - where Bill Gates is talking about his relationship with Warren Buffet. The quote below is pretty clear in that charity had always been his ultimate goal.
"Warren and I share certain values. We both feel lucky that we were born into an era in which our skills have turned out to be so remunerative. Had we been born at a different time, our skills might not have had much value. Since we don't plan on spending much of what we have accumulated, we can make sure our wealth benefits society. In a sense, we're both working for charity. In any case, our heirs will get only a small portion of what we accumulate, because we both believe that passing on huge wealth to children isn't in their or society's interest. Warren likes to say that he wants to give his children enough money for them to do anything but not enough for them to do nothing. I thought about this before I met Warren, and hearing him articulate it crystallized my feelings."
OR he was investing his money in the area that he knew best and that would have the best profit he knew how to achieve so he could donate _more_ later.
Exponential growth is king.
If he had donated all he had in 1997 he wouldn't have even come close to what he's donated today.
