It finished the season a respectable #28 in the ratings (https://en.wikipedia.org/wiki/Little_House_on_the_Prairie_(TV_series)#Broadcast_history_and_Reception). There were still more stories to tell, even though Ma and Pa were longer on the show.

Because it was terrible. People tuned in for years to see the Ingalls family. But no-one named Ingalls was even on the show anymore, and the scripts had deviated so far from the show that the series was unsalvageable. The logical conclusion was to blow up the town and pretend that last season had never existed.

I couldn't stand Almanzo's brat niece, Jenny, played by Shannon Doherety.

Because it sucked balls

I am wondering if Michael Landon simply wanted to do something else, like Highway To Heaven.

I am wondering if Michael Landon simply wanted to do something else, like Highway To Heaven.I have read that

Michael landon had wanted to leave as far back as season 4 but with nbc in the rating celler they kept asking him to stay because they did not at that time have much to replace it with

I couldn't stand Almanzo's brat niece, Jenny, played by Shannen Doherty.

I can't stand Shannen Doherty. Neither can Alyssa Milano.