Because the Republicans, with their allies in the Federalist Society, have been planning this for decades.

Because the US Constitution excludes the most representative institution (the House of Representatives) from the appointments process. The President (determined by the Electoral college, which gives disproportionate weight to low-population red states) nominates justices, who are then confirmed by the Senate (where low-population red states have disproportionate power).

And here we are. Where the conservative supermajority in the USSC exists because two presidents who lost the popular vote have nominated justices who were then confirmed by a legislative body representing a minority of Americans.