Everything you have seen in City 17 and the Half-life 2 universe comes from the mind of Viktor Antonov whose art direction for 6 years shaped the game into what it became. Until now, we never knew why Viktor decided to move on from Valve but he has revealed his reasons in a recent interview with Eurogamer.
There has been a feeling that Valve burned themselves out on producing Half-life titles after the Orange Box and since have focused on establishing new IPs and developing Steam in order to grow. This being something which Viktor wasn't keen on.
Viktor also ruled out returning to Valve in the future, even for something such as Half-life 3, as he's sitting fairly comfortable these days overseeing all visual design for Zenimax owned studios but he did admit his "best schooling in everything I did today is due to working with Gabe Newell and making Half-life 2, because this was my second education pretty much." For me, he will always remain sadly missed. Eurogamer.