FP+ was "free," but had other costs, like getting up at the crack of dawn to book attractions 60+ days out for popular rides. Many guests were frustrated by the planning required or never understood it. Also, many guests thought you had to pay for it, even if it was free.
While FP+ was all fine and dandy for people who hang out in Disboards and use spreadsheets to plan out visits, the average somewhat rich dad who dropped $10K on a trip was probably not happy about waiting in line three hours for Flight of Passage.
"I just want to go to the park, BUY my front-of-line pass, and waltz to the front," is probably what he told some cast member.
"Why are we making our most important guests miserable and leaving all that money on the table," is probably how the conversation eventually turned at Disney HQ.
In the end, if you are going to have system to cut the line, how do you decide who gets to cut? Money is the easiest, most profitable way to do it.