ML is milliliter, the unit of measurement. When the unit of measurement is represented by letters, the letters of the unit symbols are usually lowercase. When the unit comes from a person's name, the first letter of the symbol must be capitalized.
Only unit of volume "L" is special, and this symbol can be written as uppercase L or lowercase L.
This is because the symbol of "L" is lowercase at first, and it is difficult to distinguish lowercase L from Arabic numeral 1. Later, the International Metrology Congress made a resolution that the symbol "L" can be written as a capitalized L, so that lowercase L can be used with capitalized L.