In principle, whatever hypothetical 4E D&D splatbooks which could have been released in early-2011, would have first been designed and developed a year or so earlier (ie. late 2009).
So what events back in late 2009 cause a significant diversion in labor from producing 4E D&D splatbooks?
Possible candidates are:
1 - Firing of several D&D designers/developers back in December 2009 (ie. Heinsoo, Bonner, Sims, etc ...).
2 - Planning and designing of 4E Essentials.
With the firing of staff, they could have just hired some freelancers to do the work.
The planning, design and development of 4E Essentials, doesn't completely explain why there's a four month gap in 4E splatbook releases during late-2010/early-2011.
Somebody with a more conspiratorial mindset, may argue that Mearls has a secret project going on over the last year or so, such as revised set of 4E D&D core books remade to Mearls' vision. Instead of updating the 4E core books just with errata, they could be rewritten and reorganized more along the lines of Mearls' preferences and whatever new knowledge which has been accumulated over the last several years. (Most likely it will not be marketed as 4.5E D&D).