Before version 5.5, Turbo Pascal was the undisputed king of MS-DOS because of its speed—it could compile programs in seconds that took other compilers an hour. When version 5.5 arrived, it brought to the masses. For many developers, this was their first real exposure to concepts like classes, inheritance, and polymorphism.
Flexibility in how memory was handled.
Larry Tesler’s work for the Macintosh. Turbo Pascal 5.5 Object Oriented Programming Guide