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On a simplistic level, the energy density of hydrogen is far greater than chemical batteries. Less weight to carry the same amount of energy.

This is more complex than I summarised, containing hydrogen effectively being one issue.

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True, Hydrogen has a far higher energy density than li-ion batteries (~150x). But that doesn't show the entire picture. To store and use that hydrogen, you also need a very strong tank (cause of the high pressure it has to be stored in a vehicle) and a fuel cell stack to generate electrical energy. Taking the example of Toyota's Mirai, the hydrogen tank weighs 90kg to just store 5kg of Hydrogen (at 700bar). And this is despite being made of very expensive carbon fibre. If it was made of cheaper but heavier materials like steel, it would weigh much more. And the fuel cell in Mirai weighs 60kg just to output 114kw (155hp). Add all this up and it is a much more even (~2x, not ~150x) comparison with Li-ion battery (for reference Model 3 battery weighs 480kg). And given all this, not surprising the Mirai weighs (1850kg) more than a Model 3 long range (1725kg) despite having similar range and being far less powerful.

Skai VTOL claims to fly for 4 hours vs battery powered Ehang which is around 30m - for a hydrogen vs battery real world comparison. So perhaps there is some difference when these scale up. Their tank is made from stainless steel as well, rather than carbon fibre, so it appears that there is room for improvement there too.

https://skai.co/hydrogen-details

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