I can tell you it's not for the reasons you think.  Dr. D mentions
that his show is hard to sell because of its broad demographics, which
is true to a point, but it's even harder to sell when the person you
hired to sell your advertising isn't doing their job.  Dr. Demento got
completely screwed by the last company that was supposed to be selling
advertising on the show.

I know this because I actually tried to buy air time on the show.
Emphasis on the word tried.  When Technobabble came out I was going to
produce a radio spot and air it on Dr. Demento for however long I
could afford.  I contacted Dr. D who gave me the contact info of the
advertiser guy.  I called him SEVEN TIMES over a period of a few
weeks, leaving a message after at least four of the calls.  He never
answered his phone and never called me back.  I eventually gave up, as
I'm sure all the other advertisers who were trying to buy air time
did.

It was right around this time that the show stopped airing national
ads altogether.  I don't know if he has looked into other advertising
companies or not since then.

->Later.....Spice