Hydrogen is the most abundant element in the universe. It makes up close to 75% of all matter. With 1 electron and 1 proton, it is the simplest element. Lots and lots of it formed as matter condensed. Same for other elements with low atomic numbers - and oxygen at 8 is pretty low. It is the third most abundant element in the universe. So quite a bit of oxygen also formed.
Lots of hydrogen - lots of oxygen - lots of energy....sounds like a recipe for lots of water.