It would be a herculean effort to change Citizens United. It was a Supreme Court decision. So it would require either swapping out several justices or convincing a large number of Republicans to join in on a legislative change. The Democrats had both chambers and the White House since then and either it was still out of reach or just not a priority.

I've also heard rumors that Net Neutrality is coming back but it hasn't happened yet. We handle it at the regulatory level rather than the legislative level. So even if Biden does manage to get it re-instated, it will likely disappear again with the next Republican president. Policies like that need to be consistent or they don't really work. Otherwise we're essentially telling large media companies, "You can totally mess with competitors access capabilities but only every other presidential term." That give them plenty of time to bankrupt competitors.

My tone isn't meant to suggest that change will never happen. Change is inevitable. Any system will favor some changes over others. Powerful entities are pretty good at tilting the playing field in their favor. Citizens United is just one such example. Over time that creates an environment that favors those powerful entities over less powerful entities. It's a self re-inforcing decision in that it makes it easier for groups like Citizens United to promote legislation sympathetic to it's own power.

Which party you vote for does matter but it's not everything. Democrats where happy to join with Republicans in passing the PATRIOT act. After Ross Perot had a non-trivial showing they were eager to join up with Republicans in pushing 3rd party candidates out of the debates.