Why are there so many references in Durarara?
ASCII Media Works is the parent company for Durarara's publisher (a brand company of Kadokawa), Dengeki Bunko amongst other things. This is why most of these references to shows are referenced by their light novels instead of manga. 

Durarara!! and Baccano! were written by the same person, Ryohgo Narita. The same studio (JC Staff) did the animation.

Suzuhito Yasuda is the illustrator on Ryohgo Narita's Durarara!! light novels and also the author of the Yozakura Quartet manga.

The Durarara!! and Kuroshitsuji manga are published by Square-Enix. JC Staff likes to include these sort of references in thier shows.