Perhaps they expected it to be an internal code name just like "Panther" or "Jaguar" were used to describe OS X 10.3 and 10.2 internally, but were never used in public advertising or promotion. Apple only stepped on TigerDirect's trademark on "Tiger" used in software applications when they went public with it in the last few weeks (days?).
Well.... I'm not so sure they never were. "Jaguar", especially was used a LOT in stuff I saw from Apple, and they even used a spotted fur motif in a lot of their e-mails and ads... From memory, but I'll bet I can go back into my e-mail archives and ressurect some of that.