Because, sadly, David Tennant is a real person. Well, David Tennant being a real person is good for us, but bad for posting fics about him. ATaaOM has a rule against RPFs (real-person fanfics), so none of these stories can be posted there, even though they are set within the DW universe (as opposed to most RPFs, which are stories about the real people in the real world).

fanfiction.net also has a rule against RPFs, but they seem to be lax about removing stories in which the real person is in the fan universe, so they haven't touched any of my David stories, or any of the other David Tennant-meets-the-Doctor stories I've seen there. archiveofourown.org accepts pretty much anything.

Yup, I'm okay with following the rules, but I did ask them to make sure. I did wonder if I had changed David's name and put in an author's note saying, "In a parallel universe, Daniel Thompson is a famous actor known for playing the tenth version of the Doctor in the popular science-fiction television series Doctor Who..." if they would have allowed it. ;)