Why? Because it was honest, refreshing, from the heart, touching, said with love, understanding and respect towards everyone named in her speech.
One side of the 'controversial' argument says it was ungrateful she said the Academy 'made a mistake' basically... (Not a direct quote.) But if she feels it's so, then it wasn't malicious. Also - the first half of her speech thanks the Academy, praises it and feels humbled by her win.
The other side of the 'controversial' argument claims she, because she's white, can't call black people 'black'. Why is that even controversial?! She didn't mean it in a derogatory manner. It's just an objective fact. Coming from this new world of 'Alternative facts' these days everyone seems to be afraid of actual facts. If her black friends feel empowered and represented by Beyonce's Lemonade...why is it wrong to say so?
Her speech was amazingly touching and disarming. She was the epitome of lovely honesty...and that's near impossible to expect from a celebrity. All praise goes to Adele!
P.S. They both deserved the album of the year for entirely different reasons...it's a shame they couldn't both win.