If I have extra volume in the airframe, and I'm using it for fuel tanks, why wouldn't I use it for conventional aviation fuel and extend the range of my plane? I can use existing infrastructure, not have to worry about cryogenics, not have to worry about high pressure fluids...
We know how to use aviation fuels safely now. Although hydrogen is interesting as a potential fuel, I don't anticipate its use in the near future.
Existing aircraft have plenty of range already, and would not be able to (usefully) carry much more fuel anyway.
Existing infrastructure is much less important for long-haul aircraft. Facilities at just a half-dozen key airports -- say LA, NY, Hong Kong, Paris, Mumbai, Tokyo -- would suffice to bootstrap it. (Compare to, e.g., trucking, needing hundreds of stations.)
Aerogel-insulated tankage would make carrying liquid H2 easy and safe.
The value proposition is that a huge fraction of the expense of operating a long-haul carrier is hoisting the heavy fuel up to 40,000 ft. and keeping it up there. Enough H2 to get the same range weighs a third as much; the difference can be used for payload.
H2 can be produced direct from wind or solar when it's windy or sunny (respectively), and stores up power for peak demand, as well as for aviation fuel, so there are huge synergies in developing H2 production for multiple uses.
There is quite enough experience handling LH2 for rocketry. Probably carrying some LOX or H2O2, too, would enable flying at 60-80000 ft, for even more efficiency.
Not venting CO2 will matter when that is taxed, as it should be already.
The point would be to reduce the carbon footprint, for which currently there is no practical technology for aviation (which can make more than a small incremental reduction).
If you are just considering a companies bottom line, obviously they are just going to do what would make them the most money.
Liquid hydrogen actually has the best energy density by unit of mass than any other fuel (combustible fuel, nuclear is on a whole different level). If lightweight cryogenic storage is developed, then hydrogen powered plans could be a viable option.
