Why isn't homosexuality a mental disorder?

No, because homophobia does not fit the clinical definition of a true phobia. Homophobic people often think that they are superior to homosexuals, or that homosexuals are revolting. A phobia is an intense and irrational fear or aversion to something, and most homophobes don't have any actual fear of homosexuals at all. At best homophobia is just a bad personality trait, not a mental illness.
No, it is a state of mind. We use the Word phobia to describe an irrational, intense fear or hatred. They are only considered an "illness" in need of treatment when they interfere with a person's life. Some are more likely to be considered illnesses than others.

Homophobia is not a phobia like agoraphobia or arachnophobia. It is simply an intense dislike rather than a pathological fear. An arachnophobe would be terrified to be in the same room as a spider and may even be so afraid as to become completely hysterical. This can require psychiatric treatment (or medication) in a very severe case. Homophobes do not, generally, scream in fear when they see a person they believe to be homosexual. They simply hate them, irrationally.