Is there a broader nomenclature convention to explain why the ‘a’ in ‘chlorophyll a’ is italicized? By convention, the letter that specifies which side chain a chlorophyll molecule contains is italicized: for instance, chlorophyll a. Does this convention stem from any broader principle of chemical nomenclature, or is it more of a quirk unique to chlorophyll?
This is only a partial answer to your question.

The suffixes a and b to the name chlorophyll were most likely introduced by Richard Willstätter and Arthur Stoll. They are widely used in their book UNTERSUCHUNGEN ÜBER CHLOROPHYLL (Julius Springer, Berlin, 1913). Here, however, the suffixes are not yet italicized and appear as normal text.