My guess would be that Suikoden was never a huge seller despite the overly vocal minority of internet fans.
As far as I can tell Suikoden 3 was far and away the biggest game in the series in the US. Suikoden 3 was popular enough for us to get 4, Tactics, and 5 localized despite fairly poor sales for all those games.
But all of those games were on the massively popular PS2 with a huge install base, and the last 2 games were on the very popular DS, which got localized, then the popular-mostly-in-Japan PSP which did NOT get localized. Additionally, all the games after 3 seemed to have smaller budgets which would have allowed decent profit margins on relatively low sales.
Also, the creator of the series left after making 3, so there probably wasn't a very strong force within the company to push the series beyond that game. They basically treaded water with the subsequent titles (all prequels and alternate worlds).
This would all lead up to the PSN release of Suikoden 1, which I am guessing didn't do well enough to impress the folks at Konami. And as such, we have not gotten any subsequent PSN releases of those games and the series are more or less dead at this point.
Or, I could be totally wrong on all that. But I still own a copy of Suikoden 2 so whatever.