Because it didn't get very good reviews, it didn't do very well at the box office, and this was a very strong season for new plays. And also because Tony voters tend to have amnesia when it comes time to announce the nominees and hardly ever give nominations to shows that closed a while back and didn't make much of an impact when they were open anyway.
Also, while I actually thought jt was a good show, it really wasn't as good as August, Seafarer, Rock n Roll, etc.
It also had very little to say (a major criticism from reviewers), which is fine for some shows, but for a drama like it is, entertainment isn't quite enough.
Cyrano should NOT have only gotten a nom for costumes, though.
"He found something that he wanted, had always wanted and always would want— not to be admired, as he had feared; not to be loved, as he had made himself believe; but to be necessary to people, to be indispensable."
-F. Scott Fitzgerald's This Side of Paradise