. . . is arguably best put in the Other Technology section, but this is sort of a continuation of the Solar Salvation topic in this forum. I found this site on aerospace tech that answered my question;
In order to make a scramjet work, researchers must choose a fuel that can burn rapidly and generate a large amount of thrust. Hydrogen meets these criteria. ... (edited for space, MP)
There are also other advantages to using hydrogen as a fuel. First of all, hydrogen is extremely flammable; it only takes a small amount of energy to ignite it and make it burn. Hydrogen also has a wide flammability range, meaning that it can burn when it occupies anywhere from 4% to 74% of the air by volume. Since hydrogen is a gas, it mixes very easily with air allowing for very efficient combustion. Another advantage over hydrocarbon-based fuels like JP-8 or gasoline is that hydrogen does not produce any harmful pollutants like carbon monoxide (CO), carbon dioxide (CO2), or particulate matter during the combustion process. It is for this reason alone that many researchers have promoted hydrogen as a fuel in the public transportation industry.
Nevertheless, there are some disadvantages to using hydrogen as a fuel in aerospace vehicles. Hydrogen is not a dense fuel. At standard pressure and temperature, it has a density of only 0.09 kg/m3. Compare that to the density of gasoline at 750 kg/m3 or JP-8 at 800 kg/m3. While this low density is an advantage in terms of saving weight, hydrogen requires a large volume in order to store an adequate amount of chemical energy for practical use. Hydrogen gas is typically stored under pressure to increase its density, but even at 10,000 psi (68,950 kPa) it will contain only a quarter of the chemical energy stored in an equivalent volume of JP-8. The density of hydrogen can be further increased by cooling and pressurizing the substance to the point that it becomes a liquid, but even in this form it will need a tank approximately twice the size of that required by JP-8. (edited for space, MP)
So with liquified Hydrogen, you need a tank twice as big as a jet fuel, or other hydrocarbon.