Because the pitch count is indicative of how your pitcher is performng. Pitch count is sometimes more indicative of who's winning than the score, winning, not leading.
Right now, they're leading AND winning.
Doesn't all that get wiped away when you have a good bullpen? Matchups, I believe, are far more important than the pitch count. 100 is a made up number for "high" pitch count.
Also, isn't your night's pitch count gonna be really low if you get beat up in the first few innings and get taken out?