The following image from NASA shows the flight paths of Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2, as well as the positions of the outer planets.
It seems to me that with a little tweaking of the flight paths, it would have been possible for Pioneer 11 to visit Uranus and Neptune, and for Voyager 1 to visit Pluto. So why didn't that happen?
The initial plan was to visit all of the outer planets:
The Planetary Grand Tour was to send several pairs of probes to fly by all the outer planets (and Pluto) along various trajectories, including Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, which fulfilled many of the flyby objectives of the Grand Tour except a visit to Pluto.
For Voyager 1, scientists had to make a choice: either visit Saturn's interesting moon Titan, or visit Pluto. A visit to Titan would bend Voyager's path and make a visit to Pluto impossible.
Astronomers decided that in order to optimize their science at Saturn, they’d need an orbit that brought Voyager 1 up close with Titan. But that flyby also would put Pluto out of reach after the spacecraft lifted out of our solar system’s ecliptic plane.
“It was a pretty straightforward decision for them because they thought there was going to be a third Voyager mission that could come along and go to Pluto,” Stern says.
(see also the Voyager FAQ)
Back then, they chose Titan, and then NASA cancelled the third Voyager mission.
There's an alternate point of view in this Slashdot comment:
I had the opportunity to ask Ed Stone, the JPL Director & Voyager scientist, this question. His rather glib answer was, "well, Titan was 3 hours away, and Pluto was 3 years away - and I had to make payroll."
I haven't found a reason why Pioneer 11 didn't visit Uranus or Neptune, or if those visits were even seriously considered.
Pioneer 11 was launched out of the ecliptic plane by its Saturn encounter, that made it impossible to visit Uranus or Neptune. But it's possible a different trajectory would have given the opportunity to visit Uranus or Neptune.
Budget pressures may have played a role in not scheduling more flybys. Voyager 3 was cancelled in 1975. At that time, NASA needed money to fund development of the Space Shuttle.
Pioneer 11 was thrown out of the ecliptic by its Saturn encounter. The reason they let that happen was to see if a spacecraft could survive crossing a gap in the planet's rings. Voyager 2 would have to cross the same gap later if it wanted to visit the outer planets with far better instruments and cameras. NASA's page on the encounter says that there were two options for the periapsis of the flyby:
NASA managers had to choose where to target Pioneer’s flyby of Saturn to provide maximum support to the Voyagers. Options included a more scientifically interesting but riskier passage through Saturn’s inner rings or a less interesting pass through the outer rings but which would be a pathfinder for Voyager 2 that would have to take that same path to ensure the proper gravity assist to send it on to Uranus and ultimately to Neptune.
