I can only guess: Once upon a time we used to write by hand. It was considered time consuming. Once upon a time we used to set books in lead letters. There we

I can only guess: Once upon a time we used to write by hand. It was considered time consuming. Once upon a time we used to set books in lead letters. There were ligatures, where two consecutive letters hat a special spacing - like "fi" or "sz". If we allowed three consecutive letters, we would need to invent new ligatures. It is probably easier/cheaper to just forbid the spelling.