The origin of the term 'uppercase' dates back to lead
typesetting.
Capital letters are called upper case and small letters, lower case. These designations originated because the type was kept in two different wooden drawer cases. When the old-time typesetters set type, they placed the case with the capital letters above the case with the small letters, thus the name uppercase.
Once printers started keeping more typeface styles, it was more convenient to adopt a larger single case for both upper- and lower case letters. The "California type case" is a standard case layout you can still find in some high schools today.