They started to use opening bids and minimums when the larger prizes arrived, probably on the nighttime version. There was no need for that on the daytime show which had understandably lesser prizes. Unless later on (and this is just a guess) they decided to occasionally offer something spectacular on the daytime show as a special treat, thus opening bid and minimums.
This brings to mind another question... when the buzzer sounded for final bids, I'm assuming that was just based on time, no?