Why were some people disappointed that Kingsley wasn't the Mandarin? What would make Trevor Slattery's "Mandarin" more scary than Killian's "Mandarin"? Blowing up more stuff? Killing more people? Having a different agenda - perhaps world domination, or killing all of humanity?
Because he was something different for a change. In the first film sure we got terrorists, but ultimately it was Jeff Bridge's character who was the main villain (guy in a suit). In the second movie we had Mickey Rourke as a variation on Whiplash, he was pretty cool, but then we find out the main villain is Sam Rockwell (guy in a suit). Finally we get this movie which seems to promise us something different...and yet we end up with Killian (guy in a suit). Yes each villain has some variation and something to make him different, but what it all boils down to is a guy in a suit.