Why Is 7 Afraid of 8?

To lead into the joke, for the non-Whovians, I will list the actors for The Doctor and their Doctor number:

1. William Hartnell, 2. Patrick Troughton, 3. Jon Pertwee, 4. Tom Baker, 5. Peter Davison, 6. Colin Baker, 7. Sylvester McCoy. 8. Paul McGann, WAR. John Hurt, 9. Christopher Eccleston, 10. David Tennant, 11. Matt Smith, 12. Peter Capaldi

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Because 8, 8.5, 9