The sequel formula killed my interest in it for a long time.
No way was I gonna buy a PSP or whatever the hell kind of phone Coded and X released on. And since they were stubbornly insistent on making every single installment absolutely vital to the story, whether it was for PS2 or PSP or HP Officejet, that meant I couldn't just skip some here and there. So I quit completely.
They did eventually release the collections but it tooke a long time to get the motivation back to actually play them. I'm currently finally working on BbS though.