We're all aware of tonic sol-fa - sometimes otherwise called solfege in the U.S., but in U.K. meaning moveable doh. But the question I can't find an answer to is why sol-fa. Doh is the root/tonic, sol is V, fa is IV. Is 5 and 4 relevant, is the reason that I, IV and V are the main notes creating the three major chords in a key?
The literal answer is "because that's what the person who popularized it (John Curwen) called it". The name was presumably based on an "Englished" version of the French "solfège" and similar names in other European languages.
But the 7-note naming system (do re mi fa so la ti), which may have come to medieval Europe from Arabic, was used in parallel with an interlocking system which only named 6 notes, "ut" (= do) to "la". See https://en.wikipedia.org/wiki/Guidonian_hand for an explanation, and diagrams of how it was used in practice. "Ut" was later changed to "do" so that all the names ended with an open vowel sound, making them easier to sing.
In the Elizabethan period, only 4 or these 6 note names were used, and the notes of the major scale were named "fa sol la fa sol la mi". That naming system was used in the (Latin) tag about the interval of a tritone, "Mi contra fa est diabolus in musica".
The duplication of "fa sol la" in the Elizabethan note names also illustrates that "scales" and "modes" in the modern sense are not the only way to think about "tonality" - and they perhaps show that IV was considered to be a more "important" chord than V, which contained that pesky leading note...