LA and DC are. Dallas is playing in the renovated ballpark that the Rangers played in. Houston is playing in UH's stadium which seats 40K. Seattle, NY and Tampa are all playing in NFL stadiums, but those were the best options for those markets.
Seattle uses CenturyLink for football and soccer, so they have experience tarping off the upper deck.
Tampa is letting them use the stadium rent free for 3 years, and the WWE is holding Wrestlemania in the stadium this April.
New York is playing at MetLife probably because it was the site for Wrestlemania last April, and the only other available stadium would be the Red Bull's MLS stadium, but because it's grass, the soccer team probably doesn't want it torn up during the growing season. When NYFC builds their new stadium, the XFL may move there.
St. Louis is playing in the Dome there because that's the only place there now, but with St. Louis getting an MLS team, the city is going to build them a new stadium and the XFL will be using it.