Space probes are expensive. When your budget is limited, you have to compare what you can achieve with a space probe against what you can achieve simply by building a bigger, better telescope on Earth.
The Voyager probes cost 865 million USD, a number I believe is not inflation adjusted. Adjusting for inflation gives 3.6 billion USD. That compares to the cost of, e.g., the Extremely Large Telescope which is slightly over a billion Euros.
If you are the US government, how do you decide which project to fund? Chances are you'd ask the scientists to come up with ideas for what either project might discover, and then compare them to the costs. If the ELT is expected to produce as many results as the Voyager missions, it wouldn't make sense to fund the Voyager missions! Something like this is already what is being done: scientists propose projects, they are peer reviewed, and the consensus best ones get funded. Unfortunately the fact is, realistically, space probes are usually just not worth the money for the expected returns. It's for similar reasons that we've not sent more manned missions to the Moon or to Mars (I am confident that a manned Martian mission is possible with current technology, and the reason we haven't done it is because of $$$).
A fair question is why NASA sent out the Voyager probes in the first place. At that time, we didn't know as much about the outer planets, and going from 30% knowledge to 80% is a much larger improvement than going from 80% to 90% (these numbers are meant to be illustrative, not authoritative). Other reasons: these space missions capture the public imagination, which might make it worth doing anyway for publicity purposes (see Breakthrough Starshot, which is sending a probe to another star, something which is unlikely to be economically sensible also). Back in 1977 when the Voyager probes were launched, the US was in the midst of the Cold War. Space exploration was good for political points, making it more attractive. You can see from NASA's budget history that its budget then as a fraction of the federal budget was twice what it is today, and a few years before 1977 it was even higher (when getting Neil Armstrong onto the Moon was a Very Big Deal).
If you care about these space probes, support an initiative to increase NASA's budget. Example of such an initiative.