Bewitched was cancelled in 1972. It was actually renewed for 2 more seasons but Elizabeth Montgomery refused to go on playing Sam.
Some people said she just got tired of playing the role and wanted to do new things. You could tell by Elizabeth's acting in the last season starting from episode, "How Not to Lose Your Head to King Henry VIII".
Looking like she wants to be anyplace but here, Miss Montgomery has returned this season as if under a spell that has made her forget how to play Samantha. Sporting less energy (not to mention less make-up), the character of Samantha now seems slightly bored and annoyed and it's almost unsettling to watch this sudden decline. Especially evident because unlike past "time travel" episodes ("Darrin the Bold," "Sam Goes South for a Spell," "Sam's Old Salem Trip" ) where Liz expertly played "period" by adopting a different stance, voice and attitude, here she doesn't even try to be anything close to 1542 England. Rolling her eyes and doing excessively huge takes, Liz is funny at times and the song she sings is a beautiful highlight but in picking up the lute, she's dropped the ball only to have a gleefully QUACKING Aggie and Sargent run away with it.