I like the pre-firearms era more, so I’m not really an expert on WW1 trench warfare, but weren’t the “official” trench-warfare weapons exactly that? (The helmets were designed against shrapnel and not bullets, that I know from Lindybeige https://www.youtube.com/watch?v=KVFm1dc8luM )
Pistols and Bayonets, the latter “quickly” changing to self-made improvised “anything else” (shovels, clubs, anything a soldier could modify to help him in the close-distance brawl trench-fights usually were), so an armor which could help you survive those could seem “practical” for the higher-ups, and then the cost decides how many is distributed (if at all).
(And 'cos I’m kind of off-topic already, let’s go full off-topic! Did you know that metal-helmets which replaced the leather ones starting WW1 increased the head-wounded soldiers according to statistics? Now you know 
Details in Lindy’s video, 'cos I like him ramble on things: https://www.youtube.com/watch?v=1IQE0uZUMys )