The show was canceled after three seasons because of declining ratings even though season 1 and 2 were successful in ratings but by the third season ratings started to decline and by 1978 Hanna Barbera wanted to refresh the franchise because fans were getting tired of the same formula and they wanted something new so they decided to do a new scooby doo show and create a new character Scrappy Doo so they created a new show Scooby Doo and Scrappy Doo show the next year in 1979.
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Joe Ruby and Ken Spears.
Seasons 1 and 2 was successful season 3 was not successful it was a decline that's why it got canceled back in 1978.