Well the ESA is going to send one, but the answer is simply budget. It takes money to run one of these missions, it competes with other missions. Not only is there the space craft cost, the launch cost, and the team cost, since its going to take a long time to get out there you've got to have 50 years of time reserved on the deep space network etc etc.
A good question would be "Why are the space sciences so under funded?" and that policy question is best addressed with your legislators. Lately I've been positioning space as a 'back up plan' [1] for the Global Warming crisis.
[1] Motivating politicians is hard, especially if they can't point at what they've done and say "See how impressive is that!" Space sounds like a 'waste' to a lot of voters, so I've been taking the tack, "Global Warming is a crisis, and we don't yet know how to control the climate on our planet, we may get to a point where we are seriously thinking about moving off planet and this information will be invaluable, there is also the asteroid threat which space craft "out there" where the most likely earth killing asteroids are hiding (the Ooort cloud) would be a tremendous early warning, you might be saving the entire human RACE by funding these projects at NASA, how cool is that?!"
I have no idea how favorable this was, i.e. whether you could get the same effect today with more propulsion, but it's quite possible that we don't have the technology to do this in any reasonable timeframe today.
What can happen to our planet that would make it less hospitable than Mars or deep space? Most of the threats (global warming, asteroid strike, etc) still leave Earth with a usable atmospheric pressure and breathable oxygen, they just affect food supply (there is no food on Mars), or other things that still leave you better off than facing the vacuum of space.
I remember reading a story when I was a kid, about a future society that moved underground to get away from unfavorable planet conditions. The description was remarkably like what I'd picture as a large scale off-planet settlement -- everyone living in a rather confined space, and kids only knowing about what Earth was like from reading books and watching old movies.
If you believe the James Lovelock and the Gaia hypothesis, the environment is stabilised by multiple negative feedback loops. However, the feedback is limited and will only compensate for a certain range of inputs - if these are exceeded, the entire system could "flip" into another stable configuration. Lovelock suggests that this has already happened once on Earth; the change from an anaerobic atmosphere to the present oxygen/nitrogen atmosphere, caused by the "pollution" excreted by anaerobic lifeforms (the bacteria now mostly relegated to your intestines).
It's an interesting hypothesis with a fair bit of research ("daisyworld" simulation of global temperature stabilisation via albedo effects, the methyl iodide cycle, etc.)
I believe his current position is that we're all doomed and the "flip" is underway.
Sadly enough most people and their legislators find it hard to devote money to something that is extremely long term (Decades or even centuries). I think the best we can hope for for more funding in Space is competition from China in the form of a space arms-race.
In this case its "how history will remember them." Having talked with a number of congress people over the years in various stages (campaigning, lame duck, between elections, Etc) there is certainly a desire not to be known for a scandal or indiscretion.
We've shifted to orbiters instead of one-shot flybys. Galileo at Jupiter, Cassini at Saturn, Messenger at Mercury, Juno on the way to Jupiter now. A ten year spacecraft mission can deliver a lot more science when it's orbiting a planet (or driving around on it) rather than cruising through endless void.
(New Horizons is set to flyby Pluto, because orbiting it wasn't feasible as the dwarf planet doesn't have nearly enough mass to capture a fast incoming spacecraft. New Horizons would have had to fly to Pluto much slower if it wanted to orbit it.)
Why aren't we sending more of these, in every direction?