One of the design goals of Unicode was lossless roundtrip conversions to and from legacy encodings. Legacy Turkish used the ASCII i for lowercase and a character in the 128–255 range for İ.

In contrast, Greek encoded its entire alphabet in the 128–255 range even though, e.g., A and Α have identical appearances (similarly with Cyrillic letters).

This legacy use is also why, e.g., Thai and Hindi handle their vowel setting differently (in Thai, vowels are separate characters input in display order, in Hindi they’re spacing marks input in phonetic order¹) although both have their origins in the Brahmi script.

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1. Some vowels are written before the consonant that they follow phonetically, some come after, a handful come before and after and some are written above or below or modify the shape of the consonant (less sure about this last one—I have meagre Thai skills and almost no Hindi).