>>507804760 (OP)In most Islamic countries it is illegal to drink at ANY age.
But in answer to your question, it is because of the Temperance movement over a century ago. Which in turn was a direct result of the colossal mistake of granting women's suffrage. Back then abortion wasn't really a thing, but women did their women shit about alcohol and got it outlawed. It was eventually re-legalized, but the 21 age limit is a lingering legacy from all that.