The competition steers were in a feedlot for 100 days on a barley based ration. There is no reason they couldn't be finished on grass, but grain finishing give the certainty of gain and finish when you are working to a set date. Grass quality will vary with the season and unreliable rainfall makes it more difficult to finish for a set competition date.

Grass fed/finished is certainly a "thing" here as well. All my steers and heifers, (commercial not competition), are grass (forage) finished and sent to the local abattoir for the domestic market. I will supplement with grain (but still grazing pasture) when times are tough so I'm not a strict grass finisher. Pasture finishing works well for me, I'm not set up for full feedlot feeding.