Why does the New Horizons probe to Pluto have such as short operational window?

When New Horizons reaches Pluto in 2015 it will only study it for a few months. Why is the encounter so short? Why, like the Cassini probe, can't it stay in orbit for a few years to really study it in depth? After all, this is likely to be the only study of Pluto for some time, possibly in our life time.|||Pluto is far away. To go there in a reasonable time span the choice was made of a high velocity start from Earth orbit, followed by a slingshot pass of Jupiter. The result is a voyage lasting "only" nine years.





But the price to pay is a flyby of Pluto at relatively high speed of 14 km/s. To go in orbit around Pluto, like Cassini did around Saturn, almost all of this speed has to be lost. This requires an impossible high amount of rocket fuel, comparable to what is needed to put a satellite into Earth orbit.





So why didn't Cassini need that?





Cassini used a number of encounters with moons of Saturn's to brake. (A sort of inverse slingshot maneuvers.) An impressive show of cosmic pool! But impossible to do at Pluto, because the moons Pluto possesses are just too small to be of use.|||It is going too fast and carrying insufficient fuel to go into orbit of something so small as Pluto. Any space mission is a trade-off between payload and fuel capacity. It could be given enough fuel to slow itself down so Pluto's gravity will capture it into an orbit, but then it wouldn't have much scientific instrumentation on it to warrant staying in orbit. Even a flyby will dramatically increase our knowledge of Pluto and the Kuiper belt. Look how much information we got from Voyager, and that only did flypasts of all the outer planets.