Because numbers in Apple's lingo were generally case look, if you had a similar look you got the same number with a letter (well until Apple ran out of ideas and the 6, 7 and 8 all look the same). If we were strict about the evolution thing Apple would call it a iPhone 8s.Why? It looks a lot like an evolution of the 8 and nothing like a devolution of the 11. Is there something inherently ridiculous about the number 9 that’s lost on me?
But a stronger reason is that calling it iPhone 9 instantly places it below anything with an X on its name. That would be not taking marketing advantage of the powerful A13 inside.
Which on other hand may be exactly what Apple wants considering they sell the XR for 600 USD. They could sell the "9" for 400 USD (effectively replacing the current 8 which is going for 449).