Because it's probably the most-advance virtual machine out there.
It has a massive ecosystem, many years of testing and battle-tested devops-tooling.
It has everything.
We have protections- some based on the constitution, some based on individual state and federal laws, others based on common law precedent- that prevent discrimination against certain classes of individuals. In most cases those classes cover things that people have no control over (being born black isn't a choice, and neither is being born gay).
Being a bigot (which is what white supremacists literally are, so this is not just some hyperbolic statement) is not a protected class. No one has to let the klan show up in uniform to their restaurant, or let people use their platform to promote hate speech.
To be clear the click bade headline here really is awful- no one got banned for going to the rally, they got banned for hosting parties in the AirBNB rentals themselves. They used AirBNB to set up their hate events, and AirBNB put a stop to it.
Gay marriage is a political position, they may not have been able to not serve someone because they were gay, but following the same reasoning as Airbnb here they should not have been compelled to make a gay wedding cake.