...because Republicans are insisting on using the current rules to make the Senate dysfunctional. The latest is a pledge from eight Republicans -- Coburn, McCain, DeMint, Ensign, Johnson, Paul, Lee, and Ayotte -- to place a hold on every bill that doesn't meet five (apparently fairly vague) criteria.
Look, the point of holds is to allow individual or small groups of Senators a chance to work out their differences with a bill. Senators support holds because they each want to retain the ability to influence each and every bill. Holds allow Senators to cut deals that protect narrow interests.
What these Senators are saying, explicitly, is that they just won't allow any bill to move if they don't like it. Period.
That's asking the Senate to run by consensus, and it just won't work, certainly not in an era of partisan polarization.
Holds are an informal mechanism; there's nothing in the Senate rules that require Harry Reid to respect this sort of thing. Of course, if he doesn't, Republicans can retaliate...but there's really nothing they can do that they aren't doing already. In practice, a hold is simply a threat to object to a motion to proceed to a bill by unanimous and therefore to force the Senate to chew up time (by requiring, ultimately, a cloture vote on the motion to proceed). Fine. They're pretty much doing that anyway on most bills, and certainly on any bill that conservative Republicans oppose.
During the 112th Congress, of course, this sort of thing doesn't matter much; nothing is passing unless it gets through the Republican House, and therefore all this sort of thing does, if it happens, is to prevent Democrats from forcing votes on things that wouldn't be becoming law anyway. But while Senators have an interest in preserving their capacity for individual influence, they have an even greater interest in making sure that the chamber is at least minimally functional. I don't think a true 60 vote Senate does that, and I'm certain that a Senate which allows permanent holds on any bill that doesn't meet ideological litmus tests certainly doesn't.
In other words, Senate reform is going to happen, one way or another, before very long. On the outside, we're not going to have consecutive Congresses with unified government (House, Senate, presidency all the same party) without getting major reform by the end of the second of those Congresses. The only questions are what reform will look like (how majoritarian?) and whether it will happen somewhat earlier than that. One thing's for sure: the more that the minority party pushes, the sooner the majority will choose to act.