Dear Toothie Grin -
Good question, though you did manage to list most of the reasons Tom was so very perfect in the part in your own response.
However, I personally think the writers can take a bow too, as they were the ones who did create, these incredibly good, clattering tales; and most, if not all, of Tom's lines. Tom said once in an interview, that he really wasn't alowed to change his lines, except in the rehersals, which of course he did, all of the time: to the delight and amusement of everyone within ear shot! But when things got serious, it was totally back to the script that someone else had written for his character, the good doctor. The script writers and script editors during Tom's time in Dr Who
, did an excellent job for both Tom, and for his fans. So all of us were very well served.
And as a bonus the writers got paid!
[I would have worked for free!