That was not the allegation until very recently, as the work of people like Fred Leuchter and Germar Rudolf has made the gas chamber lie increasingly harder to maintain. Traditionally, the claim was that mass execution by bullets was too psychologically difficult on the executioners, which is why the Germans supposedly developed the homicidal gas chamber in the first place. They would also claim that Germany couldn't spare the millions of bullets required because they were needed at the Eastern Front. The strongest evidence for the existence of a "Holocaust" is the "survivor" testimony, and what those "survivors" described was homicidal gassing in fake shower rooms. They can't suddenly change the story now that the narrative has become untenable.