The beauty of an OTC tag is that it can be bought literally over the counter at any point before or during the season. Meaning, lots of guys make last minute decisions and show up to hunt and buy a tag. If you limit it to first come, first serve, you're limiting tags to hunter who are planning ahead to make that trip, i.e. more serious hunters, vs the "hey I think I'll go elk hunting tomorrow" guys who show up and buy tags. Just a thought.
My dad has hunted OTC Colorado the past 40 years, and I've hunted the past 5 years (we're NR). We always bought our OTC tags way ahead of elk season during the lottery period. Our unit switched over to lottery last year with all of the other SW OTC units, but nothing has really changed for us as the lottery is almost 100% odds. BUT now, requiring us to apply for it instead of just showing up whenever to buy it OTC has weeded out all the "last minute deciders" and guys who didn't draw elsewhere so settled on Colorado OTC.