Bill was only in Chicago for just 28 years - that's all.

He was the only member to be on that live version of "After The Love Was Gone", as Earth, Wind & Fire played the music. I'm sure it was viewed as an expendable song on this comp.
Bill once told how he was discussing the EWF combo tour with Chicago's manager. The manager asked him if there were any EWF songs that he could sing in collaboration with them. Bill said their manager had no idea that he co-wrote their biggest hit.
It was no secret that Bill was unhappy during his last ten years or so with Chicago. Both parties are better off going their separate paths. However, Chicago (the band/corporation) has a history of getting rid of members in a ruthless manner. Just ask Donnie, Peter, Danny, Dawayne, or Bill. They are unceremoniously separated from the group and are left to figure out their place in the world with hurt feelings and no closure. That's why the R&RHOF reunion could be an opportunity to smooth some things over.