The mystery of Ted and the underground chamber at his home is just one of several that Joss has tossed out over the years with the basic understanding that the fans would accept it and go on with the present story. Yes, the background stories would quite likely be of interest to at least some of the fans, but they didn't specifically contribute to the story that Joss was telling at the moment, so time and effort aren't wasted on them. That's not to say that they might not be revisited at some later date and filled out as would be needed at that time. We may yet some day finf out Ted's back story, or exactly the Immortal is, or what really happened in the year and a half between the end of Season 7 and the beginning of Season 8, or maybe how Angel got to be friends with Drogan, the Guardian of the Well of Souls. Maybe the devil will one day build another robot, and Angel will have to call the Five Brothers back from the grave to help defeat it. Only time will tell.