The Supreme Court should not be used to further a political agenda. While some decisions are inherently politicized, a judge should not be appointed to cast judgement in favor of a certain law or policy because they are politically partial to it.
Many progressives believe the opposite of that, like with the appointment of Ruth Bader Ginsburg. Rather than clarifying how the Constitution applies to laws, technology, and other issues, she used her personal beliefs to further an agenda. This is in direct opposition to why the Supreme Court was created, and to how to should continue to operate.