I don't think he *has* to be Asian. He could be white, black, Hispanic, Native American, or Asian. It really matters very little as long as he's not directly from K'un Lun. Logically, looking at the current spread of the Marvel Cinematic Universe, I'd trend towards the latter part of that list, because there are definite spots where you could use diversity, but it's clear Marvel is moving down a line of clear hierarchy.
The Asian Iron Fist idea came from
an Asian-American writer who floated the idea, which was later taken up by
other Asian-Americans looking for representation. Then you had the thinkpieces and all that jazz.
I admit the biggest issue I have with Danny as he stands is how he sticks close
to certain tropes I'd rather not see repeated. I love the character in the comics, but if you're remaking him for television, you have an opportunity to make changes. Marvel has taken them with other characters -
Vision, Ultron, Scarlet Witch, Night Nurse, off the top of my head - so why not Iron Fist?