I don't think that these players actually choose not to play. Seidenberg did that on occasion, but he was also busy most of the time anyway. Holzer last played in 2011, and always made it at least to the second round of the AHL/NHL playoffs every season afterwards, meaning he simply wasn't available since the last time he played.
Sulzer missed some tournaments because of injuries. He did play two games for Germany in 2014/15, but none this year, I don't know why.
Schubert had some sort of fall-out with one of the coaches, don't know which one, and simply hasn't been nominated since. I don't think that he ever publically stated that he wouldn't play for Germany anymore.
The defense is quite solid if everyone we have is actually available, but that's never really the case. Over the last few years there were always three or four defenders missing, sometimes more, leading to some very weak defenses. Even the good DEL-defenders aren't really young anymore, and compared to the offense there is a clear lack of depth.