Why wasn't the creation of the 3D printer a big deal to most people?

This is one of the biggest lies in the industry. It's so pervasive that many of the other answers to this question implicitly assume it.

3D printing is always, universally oversold as more interesting and useful than it actually is. For instance, this is the largest “3D printed rocket engine” I know of that’s flight-ready, and as I understand it the amount of printed parts has decreased over time because casting is cheaper and produces better results:

That’s a strong theme for 3D printing. Even with metals, it’s generally not appropriate for production runs, just low-volume manufacturing of small parts. Relativity Space may change that, but their development timeline has continuously slipped because - as far as I can tell - they initially had no idea what they were doing. Back when Relativity first started up, I was working with a group openly tackling the huge reliability issues associated with metal printing, and it seemed to be common knowledge in the industry that direct-printing finished metal parts was a bad idea. Even the rocket engine I just showed has machining and welding marks.

My best guess is they wanted to have first-mover advantage and made claims that put them under an absurdly large innovation burden.

I’ve talked about this trend in the past though - the new bit of detail I’m adding for this answer is that the industry is also much older than people want you to think it is. The invention of 3D printing was, wildly enough, less interesting than you're probably thinking, and less revolutionary than many of the other answers here imply, because it was not invented recently, and it's not a new technology. It's been around since the 1980’s:

The reason it got big in the past decade is a combination of patents running out and better technology making it more accessible than it was initially. Mostly the patents running out though, and that’s really the dirty secret here. There are some key patents holding the industry back, even today.

Once the initial ones ran out and the industry started growing, well, hyperbole and greed took over and people started telling tall tales about it and claiming things were going to happen that just were not possible in order to drum up enthusiasm and investment funding. There was generally an implication that the technology was new and exciting when it wasn't new and there had been decades of development to help figure out what applications made sense.

Ultimately, 3D printing is of limited use outside of creating highly customized or disposable parts. It used to be called “rapid prototyping" and even with the advent of better printed materials like metals, that still generally holds true. Even for high tech applications there are huge caveats - as I’ve been told, the FAA is adamantly refusing to certify 3D printed parts for passenger jets because no one can really prove they’re safe, and the SpaceX Superdraco engines people like to mention are actually redundant emergency equipment intended to be disposable:

The reason applications are so limited, and the technology is best suited for disposables, is that, by design, printed part reliability is absurdly poor. To reiterate, that is not a bug, it is an intentional feature of how the technology operates!

I've touched on this in the past, but in order to have such a flexible manufacturing process, it's broken down into many small sequential manufacturing operations. Usually in the range of 10,000 to 1 million of them. Let's say we have a medium print of 100,000 operations and each one has a 99.99% success rate. What's the chance of successfully printing a part?

(.999)(100000)=0.000045(.999)(100000)=0.000045

We're basically guaranteed to have some kind of defect. With a 99.99% operation success rate. Assuming you want a 95% chance of success, each individual operation needs to be 99.99995% reliable, which is just absurd. Unlike subtractive processes, machining defects when placing material have a high risk of directly impacting mechanical performance, and so when you try to adjust these reliability numbers to only consider “important” operations they still look extremely unflattering.

My analysis here is oversimplified and a bit conservative, but the basic behavior it comments on is a fundamental part of 3D printing. Because of the design choices that make it flexible and convenient for manufacturing things, it is hilariously unreliable.

The most surefire way to use a 3D printer is to be able to identify parts that aren't sensitive to defects, or to redesign a part to drive down the likelihood of producing a defect. But… that's a highly skilled design and engineering task that most end-users of a 3D printer are not exactly interested in doing. The whole idea here is that a 3D printer is supposed to be easier.

Not to mention that most printers benefit quite strongly from constant maintenance and tuning, even on the high end. I've fixed enough Ultimakers ($3000-$10000 printers) to know that's not just a function of consumer printers being cheap. The demands placed on these machines just make reliability tricky

When you see a list of top-level features it’s all very exciting, but there are a lot of tricky complications that kill most of that excitement. So, ultimately, 3D printers are a bit of a solution looking for a problem.

And, well, they have been for decades longer than most people realize.