Because it makes no sense. 'The Cage' design and the models  used in the rest of the series are nearly identical.
		
		
	 
"
Nearly identical" is 
not "identical."
So you are content to believe that an 
obvious difference indicates 
no change in-universe...but at the same time cannot believe that a 
subtle difference might be taken to indicate a
 greater one?
How does 
that make sense?
Further, you're content to believe they've visually retconned the 
entirety of TOS (plus "Trials and Tribble-ations" [DS9] and so on) in one great bite, without so much as a fig leaf of in-universe rationale, but 
not that they've retconned 
only the rejected pilot that went unseen by audiences—except as a Talosian illusion within a Talosian illusion—for decades? (And even when it 
was finally released officially, it was always with framing segments of Gene Roddenberry or Patrick Stewart frankly discussing its nature as a "television document" or early draft. That is, until it was visually reb...oh whoops, I mean "remastered"!)
All that aside, as for why two different configurations separated by more than a decade might resemble each other more closely than one falling in between them, there could be any number of reasons, but they would all be speculative, and likely deemed as inadmissable by Memory Alpha as the 
nitpick they would be in answer to. (And yet, there again, it would hardly be a wholly unprecedented situation in-universe.)
-
MMoM 
[P.S. -- if that came off as overly cranky, I apologize. I haven't had my breakfast yet.

]