All governing is balancing of competing options, so any decision could be attacked by pointing at the right lobby and saying "But Detroit needs the income from Mercury Lollipop Manufacturers/Low-end tourism/The Church of the Sacrifice of Fluffy Animals".

In this case, they've probably thought about and decided that almost all potential visitors will still be able to afford one of Detroit's surprisingly cheap hotels, and that the rest may not actually be the crowd you want to attract, anyway.