In 2014, there was no political will in the US political system to do anything--witness Obama drawing the red line in Syria over the use of chemical weapons, and after Assad used them, Congress said "fuck off, we don't want to do anything." Given that Crimea was by far the most Russophile portion of Ukraine (it only veeeery barely voted to support Ukrainian independence in the Ukrainian independence referendum), it was much more difficult to build a case for any sort of intervention there than it would be after Syrian war crimes, and the latter wasn't enough to gain sufficient political will.

On top of that, 2014 is the 100th anniversary of the start of World War I, which means that all of the foreign policy people had been brushing up on their histories of World War I and the causes of said war, which is of course the ur-example of "random event escalates to every great power at war with each other." I definitely felt like a lone voice pointing out that I was getting more World War II vibes, with Putin acting like Hitler: "I just want a little bit of X, it's the last thing I'll ask for, I promise!"

Also, probably more important than the US reaction here is the German reaction, and Germany was in full Russian appeasement mode really right up until Russian tanks were literally rolling across the border in 2022.

arguably and partially, perhaps a bit, but they have history to back up that reluctance.

That said, Putin's gonna do what Putin's gonna do and it likely wasn't the Crimean invasion that prompted it. It was the promise of waltzing in and just taking it without resistance.