USA Today spoke with one of the movie's directors, Glenn Ficarra, about why he chose to edit in Final Cut Pro X after many editors had abandoned the software for other offerings from Adobe and Avid:

He says the speed of the new FCPX enabled the directors and editing team to be able to cut the film as they were shooting, instead of after production wrapped. "It's a totally different way to look at the process."

Though it was released nearly four years ago, Final Cut Pro X wasn't readily adopted by the movie industry. Many editors that had used previous versions were incensed by limitations that Final Cut Pro X brought concerning editing from multiple cameras and the inability to import old projects from Final Cut Pro 7. This led to a series of updates that restored functionality, as well as a campaign by Apple to bring users back in March of 2013.

Source: USA Today; Via: 9to5Mac