SOMEONE explain this exception to the rules

下手

Okay so this vocabulary word for unskillful is read as (へた) but why?
Which historian do I have to interrogate for an easier way to remember this reading?

Welcome to the wonderful world of Kanji. Where even when you think you understand them, a new surprise always awaits.

“Hater” mnemonic is super effective against it.

There are words that come from Japanese before kanji was imported from China, and often they are assigned a single kanji. You know this as kunyomi.

When they are assigned multiple kanji with a single reading, it’s called 熟字訓 じゅくじくん. Other examples include 大人 and 今朝.

The reading is exceptional and unique to this particular assigned kanji combo. It’s not as though 下 also has the reading of へ in other contexts.