Because that's how it's pronounced in Russian, not Ukrainian. As I understand it, maps use this spelling because the correct spelling hasn't been widely popularized by the Ukrainian government, whereas the former was by the Soviet Union.
"Which" would be a better word to offer instead of "what." I know that "what" is very commonly used also, but wouldn't it be better to offer the more appropriate word?
Why does it seemingly, only sometimes allow the old spelling 'Kiev'. I underdstand its outdated but it's been accepable to use that spelling elsewhere in the course. I also agree that 'which' is a better word choice in English - though understand that is literally 'what' and gef that it's important to understand native sentence constructions.