Suikoden 2 is my favorite JRPG of all time, and possibly my favorite game of all time. However, it did not exactly sell well but the series did have steady attention.
Suikoden 3 broke the critic mold by scoring very highly with Gamespot and IGN, getting the best reviews out of any Suikoden game in the series. Looking at VGChartz sales which has no date for US or Europe it does tie Suikoden 2 in Japanese sales, so overall this would be the highest point in the seriers sales and critic wise. However, the gameplay was a lot different then Suikoden 1 and 2 so I never really got into it.
Suikoden 4 was pretty bad by Suikoden standards but was very much playable.
Suikoden 5 was a return to roots. Obviously was a bit on the easy side (at least for me) but the story was pretty damn good as was the battle system. One thing I did notice is this game sold pretty bad in Japan compared to the other games so maybe this is where it really dawned on Konami that sales were going down hill.
Suikoden DS didn't really have to do much with the main series and I couldn't get into it.
The only thing I could think of is Konami is not happy with the sales otherwise I have no idea why there is not a new one.