Because Steve Rogers isn't worthy, yet...
Being "worthy" is pretty sketchy and is discussed to death, but the scene that actually explains this best is from the very end of Age of Ultron.
Steve Rogers: But if you put the hammer in an elevator?
Tony Stark: It'll still go up.
Steve Rogers: Elevator's not worthy.
Steve is jealous of The Vision and wants a reason why he couldn't, we expect as much from Tony, but Steve feels cheated that he "wasn't worthy". During the party he wanted to wield the hammer to show off, little more. Now Mjolnir wiggles, because we know Steve is a good person, but right then and there, his intentions were less than worthy.
Steve wanted to wield the hammer for the very same reason that Thor did when he landed on Earth:
And Thor failed here too, so why could the Vision wield it? Because he wanted to hand it to Thor to get the Avengers moving against Ultron and immediately handed it off. The Vision didn't necessarily want Mjolnir, he wanted the Avengers to work together - thats what made him worthy.