When you are planning a major convention attracting thousand, or tens of thousands of people, total cost becomes an issue. Hotel tax is arguably not the defining reason, but it is on the list.
Weather, cost of exhibit space and the labor to set up booths, options for spouses, meeting space, exhibit space, air travel options, convenience to your industry cornerstone companies, etc also come into play.
Chicago, where you need a union electrician to inspect and plug in anything, (even as simple as an led light) plus high hotel taxes, is at a disadvantage. Location, space, etc puts them back in the game.