I still think it is because WP doesn't know where to fit him and not because Chen did not want it. Chen and Glenda have effectively become the lowest ranking CEC members, a cause for concern because Chen is an MP but Glenda is not and Chen was the stellar factor of Aljunied's victory. The other concern is whether WP would know how to use future Chens who are way above the typical opposition's rank-and-file types.
CSM id still new to the party. Let him learn the ropes and earn the respect through hard work and dedication. This should be the way and not some ah kua can become genderal in a few short years and then can become MP some more.
CSM unfortunately is not used as a strategist. If you have heard Low TK's thank you speeches, he attributes this overall to a Jane Leong, whom the press described as a veteran but only recently replaced Pritam Singh.
However, there is no doubt that Chen's empty portfolio stands out like a sore thumb.