The problem with hydrogen is that it is a much smaller molecule than helium and leaks from light, thin-walled containers like balloons. Heck, it can even leak along grain boundaries in metal containers.
If you're trying to keep a massive network of balloons aloft to provide wide-area internet connectivity, the logistics involved in topping up the gas mean that the frequency of top-ups required may be a greater concern than the cost of the gas involved.
Chemical (non-electrolysis) hydrogen generators are a thing, although not directly useful for larger scale needs like the fuel cell hype, they are adequate for topping up balloons.
