Own the rights?
Because the show heavily used popular songs from the time period, it would be very expensive to renegotiate the rights to these songs in order to produce DVD or Blu-Ray copies, or to show reruns on cable. Since demand for the show is considered low, it is thought it would be unprofitable to do so, therefore a show regarded by many critics as a classic is now unavailable except in the form of illegal compilations on the internet.
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