Why wouldn't it be on Steam? The other 5 AC games on PC are there. History suggests that Steam will thus offer AC4. They're probably just waiting for the information regarding the digital and digital deluxe version of the game before listing it. The system requirements, what digital goodies it has, etc. Par for the course for PC games that have a console counterpart. PC games take a back seat to console. And if AC3 was any indication, we won't see AC4 on PC until probably November despite Ubisoft saying they wanted to build a better relationship with the PC crowd and release PC games with console ones rather than with a delay. So maybe Steam's waiting for a more definitive release date on the PC version before listing it rather than listing it as December 31, 2013 like some other retailers. It'll be listed on Steam likely before the end of summer which is long before the actual release. Patience is a virtue.
As for Uplay, even if you buy the game through Steam, the Uplay client still loads up to start the game. It's Uplay that tracks your achievements. You effectively have both Steam and Uplay running in the background if you buy through Steam. You have to launch the game through the Uplay client regardless of who you buy the game from and if you buy digital or the actual DVD. The only game that doesn't have the Uplay client is AC1. The others do.