The other windows characters get fairly regular attention and cameos in stuff like the fighting games and StB. Not to mention ZUN has made all manner of spinoff works like SSiB and CiLR to expand on them, so they're doing allright. But, like Ruro said, if ZUN continues with his 'one game a year' plan, the current cast is going to get really huge really fast, and being able to develop all of them would be quite a task.
And lastly, all you pc-98 fanboys/girls can start whining when I see me some more decent Seihou fanart. Even VIVIT only has like 10 pictures of her all over the internets...