That is, of course, my point. If you only give the players one way to contribute, they'll only use that one way.
Yes, but that isn't the player's fault. Nor is it the game's fault - we are talking about a thing that isn't a full challenge in and of itself. The resolution of this is supposed to be quick and simple, unobtrusive to play.
Part of the issue here may be the context - rarely do players make knowledge checks, "just because". They do it in the context of some other activity - they need the knowledge to accomplish some goal or task. What is that goal or task? Why aren't the other PCs engaged in other portions of the activity, such that they aren't available to ask?
While I know folks have tons of problems with how they were presented, this, honestly, is part of what the skill challenge mechanic was designed to do - imagine that knowledge check is just one part of an overall skill challenge. The other PCs should be engaged in other portions of the challenge! Why are they just sitting around? They'll stop horning in on the action if they're already busy doing other cool things.