1. S5 did not sell very well because S4 was an awful game. S4 put a bad taste in everyone's mouth and many were reluctant to return to the series . 2. Suikoden is not what I'd call a franchise series for Konami. It has a dedicated fanbase to be certain, but it is a relatively small fanbase compared to the fanbase for Konami franchises such as Metal Gear, DDR, Castlevania and Pro Evolution Soccer.
3. You're right...it is all about the dollars. Suikoden is not series that lives and breathes on the lips of a lot of people, so why would Konami invest very much in S6?
All of this doesn't mean we'll never see an S6; rather, it is simply not a priority for Konami. Suikoden fans need to stay patient and quit being so overly dramatic about the fate of this series.