But why did the nighttime version even have shopping in the first place, besides "it's cheaper"?
Expectation that if they're tuning in to a show called Wheel of Fortune, when Wheel of Fortune is still on the air, that it's still going to be, well, Wheel of Fortune. A show that DEFINITELY had an established audience (being one of only three daytime games on the air on the first day of 1982 that were still on at the end of it [though Tattletales came on a couple weeks in]) and an established way of doing things. The show has
always been slow to change - plus, the prizes would make it more budget friendly.
If they'd gone to air with the cash format, I suspect it wouldn't have been as well received. As-is, the nighttime show launched, ratings went through the roof (and the daytime show's numbers went up too). That potentially doesn't happen if the show isn't essentially a higher budget clone of the show people already know.