When the Pluto Kuiper Express mission was cancelled in 2000, there seemed to be no hope for exploring the far planet (as Pluto was then classified). And that meant no good hope for a long, long time.
Many planetary scientists and Pluto fans reacted in dismay, especially as it seemed to be a case of then or never.
Pluto had reached the closest point of its orbit to the sun in 1989 and was on the outbound trek, turning colder. Scientists worried that Pluto’s tenuous atmosphere would turn to ice and fall to the ground, making Pluto a much less interesting place to study until it neared the sun again — two centuries later, when they would be long gone.
There was a second orbital consideration. The quickest way to Pluto is to take a left turn at Jupiter, using the giant planet’s gravity for acceleration, which cuts the travel time by four years. But a launch after January 2006 would mean Jupiter would be too far out of alignment to provide a boost.
New Horizons was a sort of last-minute underground desperation program on the (relative) cheap. Like a number of missions or hoped-for missions, it was affected by the shortage of plutonium for the radioisotope thermoelectric generator (RTG). They wanted 220 watts but ended up settling for 200.
The Pluto Kuiper Express was cancelled because its $500 million price tag was too high. The cheaper New Horizons had a budget of $500 million which grew to $722 million. Inflation and all, but only over a short period.