Population distribution.
There are 50 states in the US. Each state gets 2 senators. States with low populations (think, Wyoming) tend to lean republican while the few states with high populations tend to lean democrat. The end result is these rural states end up with a disproportionate level of voting power in the senate where the fillibuster exists.
Were the US population evenly distributed then this wouldn't be as much as a problem.
There's also been a massive shift in political attitude with the republicans. Back in the Clinton presidency, democrats and republicans often worked together on bills and compromises to keep the government functioning. This changed with the tea party and Newt Gingrich at the end of Clinton's presidency.
After Clinton, Obama was the next democrat president. He had a filibuster proof majority briefly but lost it + the house for the rest of the presidency after the first half of his first term. He was still working on compromises with the republican party (unsuccessfully) and in his second term when he lost the senate it was simply too late to get anything done. Mitch McConnell launched the "we won't approve anything you want to do" policy.
Now, with Biden as president, the senate is 50/50 R and D. 2 democrat senators have said "we will not eliminate or change the filibuster under any circumstance" and that's where we currently sit. Democrats do not have the votes to move anything.
It's not really hopeless. Assuming democrats turn out in the midterm to vote for democrat senators (we just need 2 more) then a lot of positive changes can happen. However, if instead they take the "nothing we do matters" approach then we are boned probably for the next 2 years and maybe even longer still.
What's really distressing is republicans are now deciding that "any election that doesn't result in us winning was stolen" and they are putting into place laws to allow them to decide that "We don't think the people voted right or the democrats cheated" so they can effectively steal elections. We are heading head first into tyranny.
I can't stress enough how important it is to vote for democrats in the upcoming elections. I get that people don't like them in many issues, but america is currently facing a take over by a party that has turned fascist.