The reasons Bethesda has stated for them not working on TES6 yet are that they have 2 non TES games in the pipeline, that they want to wait until technology has matured a bit more so that they can make TES6 very technologically superior, and that they don't want to be known as the dev studio that only spits out TES and Fallout games.
If they are not in-fact lying here, then (despite the intense pain of having to wait probably at least 8 years) I actually really respect them for this. They seem to respect the games they create more than their community's wishes - which is interesting, as it is very different from most studios whom would rather make a quick buck by releasing a mediocre game to a community with low expectations. This is, of course, relying on the possibility that Bethesda indeed means what it says and is not using this "we are not yet making TES 6" as a PR stunt or something. I do hope it comes out as soon as possible, however.