This is a great observation. A fair amount of the thinking behind essentials, and its ease of use, came from watching people (both veteran RPGers and newbies) struggle with the concept of powers.
People seemed to have a fairly easy time grokking a power for the casting classes - wizards, clerics. They have in their mind a concept of what a spell is, and that maps fairly easily to the concept of a power.
On the other hand, there were the people who just wanted to smack an orc over the head with an axe. Time and again, you'd see someone playing a 4e weapon user for the first time struggle to remember to use a power. The power was getting in the way of axe + orc's head = win.
(It would be ridiculous to characterize these players as dense or dumb, by the way. It really comes down to how you perceive the game fiction and how it interacts with the rule set. Some people start with mechanics. Others start with the world.)
The idea behind the essentials mechanics is to find the same relationship between power and spell for the martial classes. That's why you see stances for the fighter, tricks for the thief, and powers that kick in after an attack like power strike. For someone who has experienced fantasy in books, those frameworks are easier to understand. There's a clearer relationship between what the mechanic does and what happens in the game world.
That also points to why the design isn't interested in replacing the earlier classes. Many people obviously did pick them up and understand them. Why mess with those folks' fun?