Why doesn't Sheldon like Babylon 5?

Sheldon's dislike may stem from the initial rivalry that existed between B5 and the Star Trek franchise. One major bone of contention is the possible plagiarism by Paramount Pictures, who had access to the B5 "story bible" in 1989, before DS9 went into production. Similarities between the two shows have fans of the two series to accuse each production of plagiarism. Sheldon's accusations are somewhat typical of critics of B5, although in its defense the show was one of the first "story arc" driven series of its time and also made innovative use of CGI. Sheldon may also object to B5's referencing of material from "The Lord of the Rings", including such concepts as the Rangers, the "Return of the Shadow" and Khazad-Dum (transformed to Z'ha'dum in B5)

I don't think he has ever given just one reason for his dislike of Babylon 5 but when he starts on about it he seems to go on about it being unfocused but he never seems to explain the problem or if he has I've missed it

As of season 5 no particular reason has been given, but one could imagine him having another geek rival at some point in youth, who was a fan of Babylon 5 and made fun of Sheldon for his worship of Star Track.

This was answered in season 3- "It fails as drama, science fiction and is hopelessly derivative." -Sheldon Cooper

He's right.