The simplistic answer is that, up to now, compared to other planets, Pluto was too small and insignificant to warrant the cost of a space mission.
Pluto is so small and insignificant (about 1/5 of the mass of the Moon, or 1/500 of the mass of Earth) that the international science community has decided that it is no longer a planet and has been demoted top the status of a planetoid.
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehle telephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip." Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.
After an intense political battle, a revised mission to Pluto, dubbed New Horizons, was granted funding from the US government in 2003. New Horizons was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.
In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months before closest approach and will continue for at least a month after the encounter. New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI). The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering towards Pluto and other Kuiper Belt objects.
See the related link below for more information on the New Horizons mission to Pluto.
space probes have crossed Pluto's orbit, but never have visited the planet because Pluto was not in alignment with the flight path of the space probes.
All of the eight planets have been visited or orbited by space probes.
The Viking space probes visited the planet Mars
All but Pluto, which is currently being visited.
April 2008 At present the only planet in our solar system that has not been visited (either by an orbiting spacecraft or flyby spacecraft) has been the recently planet downgraded body of Pluto. There is a mission currently in motion to visit this distant outpost.
Humans have visited NO planet outside of Earth. Humans have visited the Moon, which is not normally considered a planet.Unmanned probes sent by humans, on the other hand, have already visited several planets.
aside from earth, the planet that has been visisted by the most probes, is often said to be mars, however other scientific research has also proven that venus and Jupiter had also been visisted by several probes every 200 - 400 years. mars however is often visited by probes every 50 - 100 years.
At current we have sent, or are sending, probes to all of the planets in our solar system. The probe Voyager has visited several planets, including Jupiter and Neptune, while Cassini is currently in orbit around Saturn.
No space probes have visited earth as all space probes have originated on earth and therefore cannot 'visit' because earth is it's home.
yes
We human beings originated on the planet Earth and so far we have briefly visited the moon but have not established any permanent residence there, nor have we even visited any of the other planets (not in person, we have sent robotic probes).
Curiosity opportunity and spirit