Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: a car, a dog, a teacher
* Proper noun: Ford Mustang, Spot, Ms. Smith
In the case of "Illustrator", it refers to the general category of software used for creating illustrations.