Startpage asked Waterfox to remove them as the default search because Waterfox users were costing them more than the revenue they were generating for them. There was a revenue sharing agreement between the two companies, but with no profit, the partnership was dissolved.
It's in the changelog notes: https://www.waterfox.com/releases/6.6.13/
I took that as an honest question and wondered that myself. A simple yes or no would have sufficed.