[robbedpeter]

It chemically reacts with a lot of things. Graphene coatings are being explored - powdered graphene oxide seems promising, but permeability without a contiguous layer of something that can block h2 molecules is a "laws of physics are laughing at you" level issue. If large super high quality graphene sheets ever become practical, they can make almost perfectly impermeable envelopes.

But yes hydrogen is cheap enough that refillable bottles and more leakage is cheaper than helium in terms of just the features of lifting gas. Hydrogen reactivity makes for other materials, engineering, and maintenance challenges that can be ignored with helium.