As said above, they chose to send Voyager to flyby Titan on a trajectory that got it out of the solar system for two reasons. First, Titan was more interesting. Two, noone knew if Voyager could last that long.
Truth was, JPL in fact already knew Voyagers could handle Uranus, Neptune and even Pluto: those assholes had deliberately over-engineered the probes just in case.
Well said, Arch.
As Goblin says, Voyager was what JPL salvaged out of the original TOPS "Grand Tour" proposal that Congress (more specifically Chairman Joe Karth of the House Subcommittee on Space Science and Applications) rejected in 1971. JPL hacked the design down to something that cost less than a third ($250 million and change) of the original design proposal, now provisionally called MJS77 for obvious reasons, which turned the trick. But one other cost-saving move was to restrict the MJS77/Voyager mission profile only to Jupiter and Saturn. Both *would* have a trajectory that would take them, or could take them, past Uranus and Neptune, and possibly, Pluto. But that would require funding the office for another decade, and that was more money than they were likely to get.
But Bud Schurmeier and John Casani, the original Voyager manager and deputy, had indeed worked to build two probes robust enough to last, they hoped, well beyond the designed mission life. Casanai was so confident that the probes could make it to at least Uranus that he asked JPL's switchboard for a "6578" extension - because it spelled out "MJSU."
That said, no one knew if the design really *was* going to be robust enough to keep the probes active as far out as Uranus and Neptune - that would require nearly another decade of travel in the hostile vacuum of deep space, after all. Such a feat had never been attempted before. And that was a consideration in the energetic debate over whether to send Voyager 1 past Titan or not. Well, that, and the persuasiveness of Carl Sagan. And Sagan lobbied very, very hard in private and public for the Titan diversion, before launch and after it, enough so as to overcome even the risks involved in sending Voyager 1 past Titan's south pole (which were not negligible). As Jeffrey Kluger noted wryly:
When Carl Sagan started going on about organic chemistry and the best you had to counter with was a lot of fretting and hand-wringing about 4,000 mile miss distances, there was little question whose argument was going to prevail. (Moon Hunters, p. 205).
So if you want to make this timeline work, your best bet may be to place Carl Sagan under a bus as your P.O.D.
But even so, the logic for Titan made a certain sense at the time. Titan *was* already known to be a very interesting place, perhaps the most interesting place in the Outer Solar System, because it had a robust atmosphere with lots of organic compounds, no less. And the probes were only funded to stay alive as far as Saturn, and there was no guarantee they'd still be operating in 1986 (Uranus) or 1989 (Neptune), let alone Pluto. It was only after the Saturn flybys, in 1981, that Congress finally approved extending the Voyager mission to those planets.
I do think that a Pluto flyby by Voyager would reduce the chances of getting New Horizons done, at least when and how it actually happened. Pluto would have been visited on a flyby already, and the instruments on board would not have told us nearly as much as New Horizon's will. Knowledge of the nature and extent of the Kuiper Belt did drive some of NH's cause, of course, and that would have come to the fore by the late 90's as well. But it's harder to make the case for the first New Frontiers mission to go to Pluto when it's already been visited. And New Horizons
had some considerable opposition in OTL as it was. Perhaps New Horizons will turn up some real surprise that Voyager could also have detected, and that will change this calculus - if so, we will know soon enough. But I doubt it.