Hydrogen leaks out of literally any container it's stored in, if you fill up a pure lead bottle(a 25kg bottle only holds 1 litre of hydrogen) it will be empty after 2-3 weeks of doing absolutely nothing - hydrogen molecules are so small they just permeate through any safe to handle material. Obviously planes couldn't use tanks made out of lead because of weight, so whatever material they used would leak even faster - and as hydrogen forms an explosive mixture with air, that's not a good thing.