Why are 댜 뱌 챠 탸 퍄 햐 됴 툐 not used?

Question

I am learning Hangeul and found this image on the Internet (I cannot locate the original source):

It seems like some of the syllable blocks are greyed out, which I think means it's not used. There's a thread on HiNative which seems to suggests they are not used unless it's for onomatopoeia. I also searched for these characters on Naver Dictionary, and it looks like there are usage but very limited:

• 뱌 - 2 words
• 툐 - 2 words

Why do these specific sounds (댜 뱌 챠 탸 퍄 햐 됴 툐) not exist in normal vocabulary? Is it for historical reasons? Perhaps because these sounds don't exists in Hanja (classical Chinese)?

Answer 1

Short answer: You can write "bzach" using English letters, and pronounce it, but such a syllable is nowhere to be found in the language. There are plenty of such "gaps", but generally, most of them are unexplainable.

Long answer: However, we can explain some of them:

• 댜 / 탸 / 됴 / 툐

  These syllable did exist in the Middle Korean period (until the 16th century). After that, the /tj/ and /tʰj/ clusters underwent palatalization, making them /t͡ɕ/ and /t͡ɕʰ/ (same as ㅈ and ㅊ) respectively. For example, Middle Korean 둏다 "to like" became 좋다 in Modern Korean.

• 챠

  In the process of the aforementioned palatalization, /t͡sʰj/ (like 챠) and /t͡sʰj/ (like 차) merged and became /t͡ɕʰ/ (like 차).