Perhaps he negotiated hardball: "I am the only one who understands this code or how to use it; you own the copyright, yes, but without me, it's useless, and will bitrot within years. I am leaving, and if I don't have it, I will simply rewrite it from scratch, and better, though it will take me a lot of time I would rather not spend and risks failure. So you have a choice: you can license it to me and we can share the results, or you can be a dog in the manger and pay full price for my future work. Choose."

Possible, but the way these big corporations work it's more likely that just nobody cared. Maybe he even wanted to improve his work within the organization and pitched his ideas but no one wanted hear them, so he left and rewrote the software from scratch for his own company.