The writer chose the title Justice League instead of League of Justice because (per StoneyB) it is a shortened version of the original name of the comic book, which was Justice League of America.
Justice League and League of Justice have exactly the same meaning. We also say Apple tree in preference to tree of apple(s), perhaps because the shorter expression is more concise (or less "klunky"!)
From a grammatical standpoint, the word justice here is a noun adjunct. It is common in English to use a noun as an adjective to modify another noun. This is sometimes called an attributive noun, and it always appears in the prenomial position, before the noun it modifies. This Wikipedia article explains the noun adjunct in depth.
Other examples in English are:
- Apple tree
- Toy factory
- Reference book
- Cough medicine
- Death rate
- Word salad
- Chicken soup†
† Although this phrase is very commonly used as an exemplar of the noun adjunct, a spirited discussion of this particular phrase here raises interesting questions.