As an aside, what has stopped us from sending more probes? Do we simply have all the data we need? It seems like cost should not be an issue, as something like the Voyager re-made with modern technology would be cheap to develop and even cheaper to manufacture.
Second, the outer planets missions are not numerous. They tend to be big and all-encompassing, with multiple science objectives and specialized imagers. (Not a quick flyby like Voyager.) Because they need to show science impact, they can't just pack a simple imager and some particle detectors.
Right now, the big outer planets mission is Cassini, which has been orbiting Saturn and visiting its moons, which are a varied bunch. Before that, there was Galileo, which visited Jupiter.
For context, I believe it's accurate to say that the era of large "flagship" missions was wound down years ago, and replaced by what, at the time, was called "faster, better, cheaper". (Engineers replied, "sure, pick any two".) Despite grumbling, this doctrine pretty much won out for planetary missions.
For inner planets, of course, most attention has gone to Mars, using the "follow the water" strategy.
Juno has launched this year and is on route to Jupiter, to arrive in 2016. I think we have sent fewer probes because we want to send new probes, not same old probes again and again. And new probes take time to design.
Shooting things into space remains to be very expensive going anywhere from $20M to $100M USD[1], $450M for NASA to put a space shuttle into orbit[2], or even $8K for the truly adventurous[3].
As well as the other replies, another cost aspect is the ground network needed to track and communicate with the probes. I remember several years ago NASA was aiming to reduce the network, in line with all the rest of the cost-cutting going on in early 2000s.
I suspect that pressure has reduced, because we don't hear about it as much locally as we did then (we have one of the ground tracking stations here in Australia, and I lived in its territory).
http://history.nasa.gov/computers/contents.html