NASA's biggest challenge has always been a budgetary one, balancing some pretty stiff political headwinds with their efforts to get things done. People like NASA, and they're proud of it; for decades, the Space Shuttle was probably one of the most obviously American symbols besides the flag itself. People, regrettably, aren't quite as fond of paying for NASA. Even in the heady days after Apollo 11, people thought the money could be better spent elsewhere:

> [Peter Flannigan, Nixon's assistant] also had become attuned to the reality that there was limited public support for ambitious post-Apollo space activities. On December 6, he sent a memorandum to the president reporting that “the October 6 issue of Newsweek took a poll of 1,321 Americans with household incomes ranging from $5,000 to $15,000 a year. This represents 61% of the white population of the United States and is obviously the heart of your constituency.” Of this group, Flanigan reported, “56% think the government should be spending less money on space exploration, and only 10% think the government should be spending more money” (Logsdon, 92).

Which is ridiculous, and grossly ignores the economic impacts of NASA's efforts (whether manned or unmanned). But no politicians have ever really bothered to make those arguments. During the FY1971 budget process, a robotic grand tour of the solar system[0] (Jupiter, Saturn, Uranus, Neptune, possibly Pluto due to the alignment of the outer planets) was quasi-considered and rejected due to cost. The Voyager program came out of that debate, at least. But FY1971 was a nasty year for budgeting, and NASA's budgeting in particular:

> NASA had been caught up in a chaotic confrontation between budget choices and broader fiscal considerations, reinforced by a breakdown in the White House policy-making process. That chaos obscured a stark reality—that through its decisions on the FY1971 NASA budget, the Nixon White House and ultimately the president himself had significantly reduced the priority of the space program among the whole range of government activities. In the form of modest funds for continued study of the space station and space shuttle, NASA’s hopes for the future were still alive, but just barely (99).

That's from John Logsdon's After Apollo? Richard Nixon and the American Space Program,[1] which is a fascinating glimpse into some of the challenges NASA faced.

You're right that, had we aggressively pursued--and funded--unmanned space flight, we likely could have done more science. But realistically, that was never on the cards. The problem with counterfactual scenarios is that we're often comparing them to an idealized scenario that relies on hindsight bias. In 1971, nobody at NASA genuinely thought we'd be limited to LEO for decades afterwards. Likewise, it's easy to look at STS and how it shaped manned spaceflight, and then come up with a list of better policy alternatives that could have been pursued with the same money.

That ignores how the program's original approval was tied to specifics of the Space Shuttle. The program's approval was, in large part, due to NASA's acceptance of major engineering changes to gain support from the military and intelligence community though most of the capability they demanded was never actually used. Likewise, the expected cost savings of a reusable space plane were fundamental to its approval. Those cost savings never materialized, and in fact things went the opposite direction. We know that today, and there were clues even enough early on. But while there were probably better manned spaceflight options, STS--flaws and all--was able to build a wide enough base of political and institutional support to get approval. Other post-Apollo options didn't and couldn't.

But the biggest factor, I think, is that manned spaceflight has cultural meaning. Politicians might eye NASA's small fraction of the federal budget covetously as something they could probably get away with raiding, but to date no one has had the guts to kill our manned spaceflight program outright. Probably because nobody wants to play the bastard who tells kids "sorry, astronauts aren't an option now" or destroy those childhood dreams adult voters remember fondly. Even if those adults now think money could be better spend "here at home" (as if we're spending it "out there" at a Space-Walmart or something...sigh). Instead of crowding out unmanned opportunities, manned spaceflight served as a sort of umbrella to give them just enough political protection to make it.

Absent those perceptions, I think there's a damned good chance that the alternative to manned spaceflight wouldn't have been unmanned spaceflight. It'd have been no spaceflight. Maybe not right away, but eventually. Perhaps that could have changed had there been a president along the way willing to genuinely fight for NASA and was willing to spend the political capital needed to persuade enough voters to recognize the economic and scientific benefits offered. Unfortunately, the electorate was never interested in it enough and politicians never saw fit to try and change that. Particularly because the political benefits of successful space missions generally accrue years after the politicians who initially funded the program are out of office. Compared to that, a bridge with their name on it is practically an immediate win.

0. https://en.wikipedia.org/wiki/Grand_Tour_program

1. https://www.amazon.com/After-Apollo-American-Palgrave-Techno...