Because he was officially hired to play the Doctor in the official continuation of the television series. Sadly, only one television episode ("Doctor Who", the TV Movie) was ever made. However, following this the McGann Eighth Doctor appeared in the Doctor Who Magazine comic strip for 9 years; had his own run of 73 BBC Eighth Doctor Adventures novels; also appeared in various other novels (e.g for Virgin Books, Telos Books, and the BBC's Past Doctor Adventures); had his own Radio Times comic strip for nearly a year; has appeared in about 70 Big Finish Audios (not including several appearances Big Finish Short Trips, both audio and text); has appeared in various BBC Short Trips (audio and text); has appeared in the BBC's "Shada" (webcast); has appeared in other Doctors' comics, the Tenth Doctor's "The Forgotten" (IDW comic story) and the Eleventh Doctor's "Prisoners of Time" (IDW comic story); has been referenced several times in the New Series (including but not limited to "Human Nature"/"The Family of Blood", "The Next Doctor", "The Name of the Doctor", "The Day of the Doctor"); and was recently seen in his own mini-episode entitled "The Night of the Doctor". All official BBC listings etc. name him as the Eighth Doctor. And, thanks to the three regenerations seen in the TV Movie, "The Night of the Doctor" and "The Day of the Doctor", it'd be impossible to remove him from the continuity, even if you wanted to.