They don't. Left field fills up far faster. The LINE into the RF bleachers is much longer, generally, however because it is closer to the Red line (CTA train), hense most people come from that direction and get into the first line they see. EDIT: and it's true you need to be there when gates open (2 hours before game time) if you want a seat in the first few rows. You generally need to be there at least an hour early to get a bleacher seat at all. If you don't get there till game time don't expect to find anything more than a single seat scattered here and there.