I have never been to KC; but, I have been told downtown is not as nice or as big as downtown Boston. Plus, Boston has both a subway network and a commuter rail network, which makes it easier for suburban families to catch a game in town. KC does not, so I can see the need to have KCs' stadium it in the burbs with easy highway access.
Before Gillette was built in Foxborough and even before talk of moving the Pats to Hartford or Providence, there was talk of building a new football stadium in the Seaport district before it got big by the cruise ship terminal. NIMBY's shot it down. A new Fenway was also targeted for the area, except closer to South Station, which would have been awesome (think the SF Giants stadium); but, McCourt lost his bid for the Sox (and got the Dodgers instead) and that idea died. The Seaport has since boomed and no way can anyone afford to build a soccer stadium there now. Somerville was begging for the stadium to be built next to the new Assembly Square development, which includes an new subways station; but, not sure if there is any land left now. The other location is in in Revere at either the horse track or the old dog track right on the water. Yes, parts of Revere can be a dump; but, a location on the water adjacent to the subway and near the airport is very good. Even without the casino that may now go to Everett, such a venue could serve as a catalyst to make that area nice along the lines of Jersey City (awesome along the Hudson, less so on the other side of the hill). Both of those locations would be better than talk of where Boston's Olympic stadium would be - in between Roxbury and Southie in an old rail yard, which would be isolated as it is surrounded by I-93 on one side and active rail lines on the other. The land there is cheap, while building over the highway and rail lines to connect it with Boston would be expensive, and it would be at least partially publically financed due to the Olympics. It’s a developer’s dream. Kraft is a developer.
In 2014, the four teams with the lowest attendance are Chivas (7,000), San Jose (14,900), Colorado (15,000), and Chicago (16,000). Chivas has unique (hopefully) issues. San Jose is getting a new soccer stadium next year. Unlike all of Chicago’s other pro sport venues, which are in the city and have subway access, Chicago's pitch was built in an industrial, inner suburb with no mass transit connections (one has to take a shuttle bus from Midway Airport) and no direct highway connections. No wonder the Fire do not draw well. Denver’s pro teams are all downtown, too, except for the Rapids, who stadium is built somewhat near a high to the east of the city (which itself is a problem as most of Denver’s population lies along a north/south axis from Denver, not east/west) on an old re-purposed military base with no mass transit service. A good counter example is DC United. They are not the powerhouse they used to be and are located in a dump of a stadium; but, it’s in the city with easy access to the subway and thus they draw 17K, which is average.
Thus, I think the lesson is that if the city has a thriving downtown with public transit, the city’s MLS stadium needs to be downtown. If not, then the stadium should be in the burns next to a highway and preferably part of a larger development. Boston clearly has one of the better downtowns in the country, thus, the Revs should be in the city.