The answer is because what we have here is a conflict of narratives. And when the truth gets obscured by "official" attempts to impose a narrative on events, this is what happens. Fortunately (I am being sarcastic, in case anyone wonders) we have Al Qaeda to help clarify things for us. Their message to would be terrorists: "Next time kill straight white people so it will be clear you did it for Islam instead of some other motive that the Left wants to impose on the situation."
National Review's David French had this to say on the subject:
This would be hilarious if it didn’t signify how twisted our world has become:
Lone wolf jihadists should target white Americans so no one mistakes their terror attacks for hate crimes unrelated to the cause of radical Islam, Al Qaeda writes in the latest edition of its online magazine. In an article first reported by The Foreign Desk, Al Qaeda of the Arabian Peninsula (AQAP) called for more self-directed Muslim terrorists to kill in America. But the article, titled “Inspire guide: Orlando operation,” tells terrorists to “avoid targeting places and crowds where minorities are generally found” because if gays or Latinos appear to be the targets, “the federal government will be the one taking full responsibility.”By attacking a gay club, Omar Mateen likely thought he’d chosen the perfect target for illustrating his jihadist motivations. After all, ISIS and multiple other jihadist organizations are known for executing gays in particularly vile ways. But when the Left’s narrative collides with ISIS’s, the Left’s narrative wins — so a Muslim Democrat’s attack became an expression of Christian Republican hate. It was the most extraordinary perversion of truth that I’ve ever seen.
Jihadists will keep attacking, but some leftists will keep excusing. It’s a deadly race to the bottom, with each leftist excuse only postponing the day when Americans finally remember what it’s like to fight the enemy, not each other.