Supply side #1: There are fewer scandals to investigate because the Obama people are unusually ethical and good at their jobs. I'm very skeptical of any such explanations. Sure, it's possible, but I tend to assume that there's always some scandal, somewhere. And at any rate, I'm fully confident that creative Members of Congress can invent one.
Supply side #2: The Obama White House is unusually competent and/or good at information management, and so no one is leaking stuff that could become a scandal. There's surely some of that, at least compared with the initial Bill Clinton WH. Still, again, I lean more towards demand-side explanations.
Demand side #1: There's something about Issa. Perhaps Oversight and Government Reform Chair Darrell Issa is simply not very good at it. There seems to be a bit to that, but I don't think it's a sufficient explanation, because other committees could certainly step in and hold hearings on their own scandals if Issa wasn't doing it.
Demand side #2: Totally, totally, speculative, but here goes. Perhaps the reason that the House isn't manufacturing scandals is because the changed media environment has changed the incentives. In the old days, opposite-party Congresses had to work hard to manufacture scandals good enough to get the neutral press to notice. Now, why bother? Most partisans, and especially the primary voters that Members of Congress are increasingly most worried about, get most of their news from the partisan press, and they don't need any Congressional stamp of authority to consider something a legitimate scandal worthy of devoting hours of programming to.
I suspect it's #4. How could we tell? I'm not going to do it, but what someone could do is to monitor the partisan press (Rush Limbaugh, Fox News programming, and on the other side Rachel Maddow's show, the liberal blogs) and see to what extent their focus on scandals is related to NYT-certified scandals. Or, we could test awareness of scandal stories by party identification, or by media consumption, and see if it changed over time, with much bigger gaps now. I don't follow public opinion research closely enough to know if anyone has done that over time..I know there's some Pew stuff going back a ways, but I'm not aware of any comparison to the 1990s or 1980s or earlier.
If it doesn't seem to matter to the partisan press whether the rest of the media is paying attention or not -- or if they're even more dedicated to it if everyone else is ignoring whatever they're on about -- then that's a very different world than one in which they sit back and wait for a "real" scandal to emerge. And it's a world in which Congressional hearings, which tend to force the Times and the rest of the neutral press to notice, just don't matter as much.
Now, even in that world there's still a strong incentive for individual Members of Congress to hold high-profile hearings, because they want the publicity. But there's a lot less incentive for partisans in Congress as a whole to push for it, and so if one committee chair is underperforming no one is going to care very much.
So my guess is that it's about the partisan press and changed incentives. But again, it's really just a guess.