Why would he sell it on sites like GOG when he will always cater to a very small niche who will buy his RPGs no matter what, and he can make more money by selling the game himself and charging more? It's like Matrix games. He's doing it right.
Unless of course being on GoG gets him much more exposure. But that remains to be proven. It entirely depends on whether GoG, or others like RPS, Codex, Kotaku (...), communicate on the fact that GoG is listing KOTC. It looks like a gamble to me unless marketing is agreed upon during discussions.
KOTC may be an old game not getting anymore coverage but if KOTC2 gets some it will benefit both games so unless Pierre gets the assurance that GoG will actively help him sell, as opposed to just quietly listing him, slashing the price to get on GoG might turn out to be a very bad idea.