If you've paid attention to their interviews over the years, a lot of the older members of Valve burned out hard working on HL2. It wasn't a remotely pleasant experience in the end, particularly The Hack and all the surrounding drama about it. And then expectations just kept getting jacked higher and higher with the episodes, and you had this frankly kind of uninspired-sounding outline for Ep3...
If I was Gabe & co., and I discovered I'd cracked the code on digital distribution and had provided a platform for tons of developers, big and small, to reach people, and simultaneously began supporting this young team that then made something interesting (Portal & Portal 2), and then also began supporting the creation of the updated version of one of the most popular games on the planet (Dota2), you bet I'd drop my old project with a lot of painful emotional baggage attached to it like a hot potato and not look back. They maybe could've been a little better about communicating how they felt, but it's not hard to see how things evolved the way they did.