Default Immersive mode in apps

While this question is not directly related to Nexus 5, but since it is one of the few devices running KitKat, I'm posting it here. Its been more than a month since KitKat has launched, but even today the "immersive mode" (which I feel is SO important!) has not been translated to many apps that I use regularly -- Kindle, Pocket, Feedly etc. Its not like these apps haven't updated in the last month or so (they have). Is there any reason why these companies have not considered putting immersive mode as a priority item?