Perhaps it has to do with the shear volume of Helium that commercial diving companies consume and the cost reductions for buying in-bulk. What we used was purchased specifically for scientific testing and as I recall, the cost was about double the cost of what we were paying for Helium at the time.
Neon is too heavy to breathe in deep diving, so it's diving envelope is more restricted. OSHA would likely go through the roof if you used it commercially in the US, as there are no proven Neon tables, to the best of my knowledge.