Howling:Why is earth so vulnerable?

Why is earth the most invaded/visited planet in the universe? Has there ever been an explanation. The only in universe reference I know of is when the second doctor is on trial by the time lords, he says earth seems more vulnerable than others as a reason for why he had had frequent visits to it.--Coop3 ☎  18:56, October 5, 2013 (UTC)

Do we know it is the most invaded/visited planet? I thought that was David Walliams' character's planet. 87.102.91.126talk to me 19:38, October 7, 2013 (UTC)