CBS has been on a rampage over the last few weeks. The network has canceled several popular shows, including some that have been on for nearly a decade. Fans have been taken aback by the swiftness of CBS' decisions, especially coming off the heels of the controversial Blue Bloods cancelation.
The network has scrapped SWAT (for the third time), as well as the FBI spinoffs FBI: Most Wanted and FBI: International. The latter two has been particularly surprising, given that the flagship show is still in business and has already been renewed for additional seasons. Fans have naturally inquired about the reason behind these sudden cancelations, and they actually differ depending on the show.
CBS allegedly shifted its FBI priorities

Let's start with FBI: Most Wanted and FBI: International. The network has not issued an official statement as to why they chose to scrap two thirds of their franchise, but several sources and insiders have weighed in on the decision. The most likely cause behind the cancelation is CBS wanted to free up space to take the franchise in different directions.
The network is planning to move forward with FBI: CIA in the coming year, and given that they only have three time primetime slots in the evening FBI airs, the decision was reportedly made to cut the spinoffs that were not performing as well. This widely accepted reasoning many suspect that only one show would be canceled. CBS basically decided to start from scratch, however, by canceling both.
Deadline also revealed that CBS planned to cut costs for both Most Wanted and International in the months leading up to their cancelation. The outlet took this as a sign that the network was shifting their priorities toward other programming. Logically, the cheapest way to save costs is by canceling the show altogether. A similar series of events led up to the cancelation of NCIS: Hawai'i.
Why did CBS cancel SWAT?

SWAT is a bit more complicated. CBS has made it clear via various statements, that it likes the show and would like to keep it around. Hell, they have canceled it and then revived it twice now. But it has come down to the inability to strike an agreement with Sony Pictures TV, the studio that owns rights to SWAT.
An insider previously told Deadline that CBS was able to revive the show because "producer Sony Pictures TV was able to propose financial terms that were appealing to CBS." This time around, it didn't go as smoothly. "CBS did not engage in renewal talks," the outlet stated. "Opting to end the series. SWAT showrunner Andrew Dettmann described the news as "heartbreaking."