As I said above, the mentality behind why/how things were done and approached changed.
I haven't played 4E in quite a while, so I'm admittedly out of touch with the details.
However, the changing motivation and mindset behind the hows and whys of the game could be seen in how encounters were designed, adventures were written, mechanics were designed, and implied expected playstyles.
A similar thing could be seen in very-late 3.5. While technically the same game, mixing and matching elements produced varying results.
Certainly, a game should be revised and improved as flaws are noticed -as was the case with monster math.
In other cases, change for the sake of change ranged from causing new flaws to just not being necessary.
For example, the ever evolving Skill Challenge Guidelines never quite seemed to work, and a lot of Official advice on how to run Skill Challenged (and the game in general) was bad advice, which somehow got worse as time went on.
I felt that Skill Challenges were a good idea, but it took learning that I should ignore the "official" advice to get the best experience from them. That relates to Essentials in that the revised guidelines and advice went even further in a directing that I felt was bad.
Mechanically, the game was adjusted to better cater to an outlook that became increasingly at odds with what worked best at my table. Essentials was the pinnacle of that conflict.