WebKit did start with KHTML as a base, but it was Apple (and later Google) who brought it to prominence. When it was only KHTML it was a bit player.
As for market share, that's percent of devices sold. By web browsing it's about 25%, and that doesn't include the possibly outsized influence it has due to being so much more common at the top of the market.
Re: iOS being popular; You're in a bubble, the iOS market share as of 2016Q3 is 12.5%.