Back in the day wasn't it more or less who had enough courses in the area to accommodate all of the players? Back before pro and am worlds were split into unique events.
I was still at Iowa State in 2004 when Des Moines hosted worlds. It was awesome seeing the pros play Stable Run in Ames, which had just been built a couple months prior. I volunteered to spot a couple times throughout the week.
I still think those 2004 super rocs were some of the best worlds fundraiser discs ever, aside from the 2006 Augusta wraiths.
Pro and Am Worlds were only combined a few times. I think Ann Arbor in 2000 was the first time they did it. Des Moines in 2004, Kalamazoo in 2008, Kansas City in 2009, and Charlotte in 2012 were the only other times they did that.
I had a lot of friends that went to Worlds in 2009, but I remember two in particular. One guy played Advanced Grandmaster and had a blast. He went on and on about Barry Schultz remembering him from another tournament they had both been at (hint: if you are weird enough that a traveling disc golfer remembers you, that should be a wake-up call :| ) and being at the playoff ending had him all jazzed up about disc golf. The other guy was a low-level local pro who played Open. His basic summary of the event was "The PDGA cares more about the flymart than they do the Open players."
Which was always the ying/yang of the old World's and NT events. Am's generally loved being where the Pro players were, playing in the the same events, going to the same players parties, etc. Open players tended to not be into the experience; they wanted bigger payouts, professional events, etc. Not hanging out with dudes playing Advanced Grandmaster didn't seem to strike them as a downside.
Pretty much everything that would have been considered the "tradition" of holding PDGA Worlds in 2009 is gone now, though. You don't have multiple divisions so it's not the disc golf gathering it used to be. You don't use a bunch of courses. The number of rounds are reduced, so you never play two rounds in a day and have to hustle from one course to another. The entire look and feel of the event is fundamentally different.
Except you are still taking hosting bids pretty much the same as when Worlds was more of a big party for disc golfers and oh by the way Climo wins again. I'm not sure that works anymore. Actually it never worked all that well, anyway.