Because it’s true. Here’s an article from Tim Sweeney from 2001:
https://web.archive.org/web/20010105180900/http://www.gamespy.com/legacy/articles/devweek_c.shtm
Mainstream application programmers switched to C in the early 80’s. Game developers were slower to switch, because their small teams and focus on performance kept assembly language viable till the following decade. When id Software released DOOM, they surprised much of the industry by having no reliance on assembly code–despite excellent game performance, and by successfully cross-developing the game (in NeXTstep and DOS), then successfully porting it to an astounding variety of platforms.
Those games didn’t have the splash that Doom did for this sort of thing.
https://web.archive.org/web/20010105180900/http://www.gamespy.com/legacy/articles/devweek_c.shtm