My guess,
So based on this thread, Natasha Otomoski probably Satoshi Nakamoto, number 21 probably max supply of btc, based on coinmarketcap max btc supply is 21,000,000 which is have 21 too.
It is possible to turn the question into "Why Satoshi Nakamoto Create bitcoin with 21 million supply?", Its pretty similar tho, "Why The Comb Of Natasha Otomoski Has 21 Teeth?" , but idk, just guessing.