Because from what I understand, the WH cleaning staff have been their most of their entire adult lives and some are second and third generations of their family working there and they have a code against that sort of thing even though they could probably make some serious cash writing a tell-all.
Which... leaves what? Everyone else? Security guards, non-senior adviser, senior adviser, and even whomever opens the door to the WH.
What makes you so sure it was a janitor or that Woodward just made it up?
Logic, but most importantly:
1. Bob Woodward is now in a position where revealing the source is the
only possible way to validate the veracity of his story.
2. If he reveals 1. who it was and 2. shows the e-mail it was sent from, then the issue of validity is put to rest and an investigation can begin on why a senior WH official is going around threatening journalists.
3. If he doesn't, and it's most likely that
he won't, then the question is: Why not?
The only logical conclusion to #3 is: There is no source and he made it up.
Why?
Well, the source, by threatening him, has made it obvious that they won't give him any further information so revealing it wouldn't change anything in regards to his relationship with said source. If anything, it would give him some leverage. However, he wouldn't be the first or last journalist to ever come out with government attempts to stifle the press so why all the mystery and hush, hush? Well obviously, there is no source or he's highly inflated his claim
AT BEST.
The only straw here is the nonsensical claim that this couldn't have been just an old man trying to milk another 15 minutes of fame out of his expired journalistic career.