Raps can offer him an extra $50 million. That's a lot to turn down. Raps could also offer him load management, sitting him 15-20 games without really hurting the teams chance at a high seed or playoff spot in the East.
Also, from the looks of it, Leonard doesn't like the media attention/spotlight, so Toronto is a better fit than a market like LA or New York. Could be a significant factor as well.
As long as he knows he has a chance at winning an NBA Championship in Toronto for the next 4-5 years, get a good amount of rest during the season, I don't know why he wouldn't want to stay. Best chance to get to an NBA Finals is in the East IMO, especially if the Bucks can't keep Giannis after.