They likely gave up because they realized that trying to impeach him is just adding fuel to his bases fire.
You gotta realize that Trump has a third of the country believing that he is a victim of political persecution. The "witch hunt" rhetoric was taken hook, line, and sinker. They sincerely and earnestly believe that Trump is a good man with a righteous vision, who is targeted by "the radical left", which is "weaponizing" the DoJ or the impeachment process.
But if we apply this logic then Democrats should never oppose Trump or any other Republican, and where does that leave us? I find this quite ridiculous as not only does it excuse Democratic inaction but also mandates that they bend over backwards to support him in the future for fear of losing their job (a job that quite literally is to represent the people).
Merrick Garland lost his shot at SCOTUS because instead of fighting back when the law was fully on their side nearly a year before the 2016 election, they arrogantly thought that they were guaranteed to win and Clinton would then get the nomination. They again backed down in 2020 just a month before the election and allowed Republicans to ram a nomination through. They backed down in Texas and allowed the Republican legislature to gerrymander districts and pick up several seats. They backed down on the government shutdown and allowed Republicans to take away our healthcare. They backed down and allowed the passage of the BBB. They're backing down and allowing the capture of a sovereign nation's president. They're backing down and allowing ICE to murder citizens in the streets.
These are all reasons why they're losing elections. They're supposed to be the opposition party yet they refuse to oppose anything and even vote alongside Republicans often enough. Refusing to acknowledge this is why Trump won in 2016. It's why he nearly one again in 2020, and it's why he won in 2024. With each passing day they look more and more like they're all members of the same party because that's the only logical explanation for what's happening. Using the same tired excuses over and over and over only gets you so far before people see right through the BS.