The reason is because it aids the reading of printed music. The different sizes of instruments produce different sounds within the audio spectrum, and the Tuba, Trombones etc would be playing notes so far below the staff that the leger lines would be too confusing to read. The same goes for Clarinets and flutes ...... too HIGH up on the staff.

The only reason for the different keys was for reading music. The strings even have clefs that many wind players never heard of.