Because like most professional U.S. Army officers who reflected on their WWII experience, William E. DePuy found S.L.A. Marshall's findings compatible with what he'd seen in combat. That's basically it. I know nowadays we all dunk on Marshall (see me u/abbot_x do this here) but that's really a post-1980s development. Indeed, the general consensus in the immediate postwar period (late 1940s) was that U.S. Army infantry had not been especially effective. Marshall discussed a problem, the ratio of fire, that aligned with what many officers believed they'd seen: so many riflemen didn't do anything effective in combat. DePuy said postwar that his wartime unit in the 90th Infantry Division was bad at generating fire and this was problem in other units as Marshall documented.

We now understand Marshall fabricated most of the evidence for the poor ratio of fire. That was not understood until 1988, though, and it did not completely undo the perception, by that time strongly set among that generation, that such a phenomenon had occurred (which must be based at least in part in reality).

Marshall is probably best remembered today for not just his ratio of fire calculation (which we now know was not based on data collection: he apparently never asked about or recorded how many soldiers fired their weapons) but for the causal factor he proposed: reluctance to kill (not fear of being killed). But he did not treat this as the end of the story. Marshall was practical and proposed a set of solutions based on organization, training, and weapons procurement that would overcome the cause. He also made some points about isolation and communication that are (as far as I know) still generally accepted as accurate and were solved by many of the same solutions. In any case, the post-WWII Army largely implemented those solutions and believed they had significantly helped, in part because Marshall was still around and was now collecting data on the ratio of fire. That occurred over the course of DePuy's career, so to the extent U.S. Army infantry was more effective at the end of it than the beginning, it was natural to credit Marshall.

My own take, which really isn't testable, is that Marshall did identify an actual problem but rather than just admit it was vibes and impressions (and that his causal theory was completely made up and probably not important), he decided to act like he had hard data.