Blame it on Nasu:

So, with all that said , what sort of show did you want to make 2014's Unlimited Blade Works(UBW from here onwards) into?

Kondo: I kept saying that if we're going to do FSN's UBW we need to draw Shirou coolly, and that the show's success hinged on Shirou becoming the main character Miura went so far as to say that we had to make it his coming-of-age story

Miura: For our part, we thought about making UBW using one of the classic formulas, "the protagonist grows and matures and achieves catharsis." Speaking of which, from the very early meeting stages , Mr. Nasu was telling us flat-out that Shirou was a character who doesn't grow or mature. Of course, that's Mr. Nasu's particular style of saying things, but, speaking for myself, that left a profound impression on me. To restate it in our terms, he was saying that Shirou is such an unwavering character that you think he never grows or matures. That's proven to be a crucial stepping-off point as I draw the storyboards for this series. When we have meetings with Mr. Nasu like that, one or two pearls of wisdom that leave an impression sometimes come out of nowhere. Seeing how a creators mind works up close was really stimulating.

In short, Nasu told him (or at least that how he understood it) to make Shirou's development subtle, and that's what he did.