The show originally ran on Fridays (season 1) and Saturdays (season 2), two dates with generally low viewership levels. The ratings for the Friday airings weren't that bad, good enough to get the show renewed for season 2, but not good enough to keep it on Friday, but the ratings went down the tube once they moved it to Saturday.
I was going to write a new Sig today, but everything changed when the fire nation attacked.

It seems like it is more appreciated now than during its original runwhich could also be said of the original 1959-64 series. I know part of the problem was that CBS aired it on Friday nights, which was a very hard slot for a fledgling show to find an audience.