I think these are both for the same reasons. Neither psionics nor essentials is necessarily weak, but neither feels "iconic" to 4e to a lot of people. Psionic classes don't show what's different about 4e compared to 3.5 or 5e, because they themselves are different from standard 4e classes. And essentials classes either aren't different enough (Slayer, Knight) or are the same as standard 4e, so they don't really showcase anything special.
I love both, myself, for different reasons. But I wouldn't put them in this list.
I suggest Rogue to show what a martial class looks like when given more than just "I stab it" as an option (as in 3.5, and some of 5e). Others suggested the Fighter, but I personally felt other tanks were more interesting, and the Rogue had the same issues in other editions.3. Perhaps I'm too unimaginative, but I don't feel the Rogue and Ranger suggestions - those classes seem similar in tenor to their counterparts in any other edition of D&D to me. Sure, there are some ripples, but unless you seriously CharNotOp (hopefully for the sake of RP), there aren't exactly a myriad of good variations, and the good ones seem to be very similar to other editions.