Space probes have crossed Plutos orbit but never have visited the planet Why?

Answer 1

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.