Because it wasn't just Heironimus making the claim about the costume.
It was also, Heironimus's mother Opal, Opal's SIL, Willa Smith, and Heironimus's nephew (8-yo at the time) John Miller. They all claimed to have seen the costume.
Heironimus's brothers, Mike and Howard, didn't see the costume, but heard about Bob's involvement in the filming.
Others, such as Merle Warchime, have seen the costume in other settings around Yakima.
Harvey Anderson, owner of a gun and camera store in Yakima, recounted his meeting of Patterson when Patterson brought in a plaster cast, supposedly taken from a bigfoot print and asked about renting a camera to film the 'creature.' Anderson pointed out the problems of the ball of the foot in the prints being too narrow to support a bipedal animal. A few days after their initial meeting, Patterson reportedly brought in other 'prints' where the shortcomings had been fixed.
Even Patterson's own brother-in-law Bruce Mondor outed Patterson's plaster casts as fake.