I guess (IANAP) that's because hydrogen is easier to observe as part of interstellar gas within a galaxy as opposed to the thinner intergalactic filaments which

I guess (IANAP) that's because hydrogen is easier to observe as part of interstellar gas within a galaxy as opposed to the thinner intergalactic filaments which is also further away and doesn't get illuminated by stars or stellar remnants. So the baryonic matter in galaxies is already accounted for to a larger extent than the filaments.