Bud Collyer's Beat the Clock also had a fair number of guest hosts, didn't it? Sonny Fox and Frank Wayne come to mind immediately, but I think there were more.
Chances are, Cullen's TPIR had many guest hosts because Bill was a busy man. He also did IGAS, Name's the Same, and other shows while TPIR was on the air. By coincidence, Truth or Consequences followed TPIR on NBC in the beginning, so in theory, they COULD'VE asked Bob Barker to guest host, considering it was on the same network.