Dear Always,
Or it may be that there is many sources available to quote, that many great seforim are rarely if ever quoted. Besides, the Bostoner Rav is not easy to quote from his seforim. His main mussar ideas are very lengthy. And his shiurim were amazing as to his clarity. A rebbe who lacks his powers of understanding, will not be teaching much by quoting him. His letters (The black sefer.) are really good. They should be quoted more. As it is probably his most popular sefer among the lamdanim.
Rav Kook’s seforim are a completely different story. His style is more artistic than explanatory. And most of his writings are not on yeshivishe topics. In a way he can be used more. The closest comparison I can think of, would be Reb Tzaddok. (I mean in how his seforim can be utilized. The differences are obvious.)