Simply, it's because musica is a feminine noun.
What is thesaurus Latinarum Musicarum?
The Thesaurus Musicarum Latinarum (TML) aims to give free access to and make searchable every known Latin text on music from the late antiquity to the seventeenth century, in multiple editions and in transcriptions from original sources.
What is the Latin word for feminine?
femininus/feminina/femininum, AO = woman's, female, feminine, pr…