Dave makes liberal use of other people’s characters and routines, especially comedians and musicians. I believe he has consistently said that it’s only fair that others can use Cerebus in the same way.
It’s an unusual stance, but he’s been consistent about it.
If I understand correctly, he is estranged from his family, he is celibate with no wife or girlfriend, and has no children, so perhaps there is also a question of whom he would give Cerebus to, if not the public domain. But I have zero knowledge of his thinking in that regard, I simply don’t know.
It’s more likely that this gesture fits in with his beliefs as a creator, is all.