Take a look at the list of exceptions. They are all high profile hotels in high profile cities. It's a scenario where the hotel chain needs the hotel more than the reverse.

For most "regular" hotels, being a part of a chain is very beneficial. Without the constant stream of brand-loyal guests, many hotels would flounder. None of that is true for hotels in high profile cities. While they might not do as well, I bet every one of the hotels on the exception list would still make a decent profit if they uncoupled from Marriott.

Essentially, Marriott needs hotels in these cities. Hotel owner know that fact and use it to their advantage. Those hotels negotiate exceptions to the franchisee agreement that would be impossible for thousands of "regular" hotels to get.

Added: This opinion is based on when I worked for Radisson at a corporate-owned and operated hotel. I worked closely with our regional Director of Revenue Manager. (I think that was her official title.) We discussed this very issue, as the chain was attempting an aggressive expansion at the time. She mentioned how difficult it was to find good hotels in high profile cities like New York and Los Angeles and that the owners have all the leverage in those cities.