It was that he rejected the US Constitution. And that would seem, on the face of things, to be a pretty convincing disqualification for the position of Associate Justice of the Supreme Court.
Writing in The Tempting of America, Bork held that where the Constitution is silent, the legislature is free to act:
In his 1905 Lochner opinion, Justice Peckam, defending liberty from what he conceived to be "a mere meddlesome interference," asked rhetorically, "[A]re we all ... at the mercy of legislative majorities?" The correct answer, where the Constitution is silent, must be "yes."
The stated original intent of those who framed the Constitution was 180 degrees opposite Bork's opinion.
The Constitution enumerates powers. Those powers it doesn't enumerate, the legislature (Congress) doesn't have -- and per some parts of the Constitution, the states might not have them either.
In other words, where the Constitution is silent, the legislature has no power at all.
Bork didn't advocate "original intent." He advocated "judicial restraint" in explicit opposition to what he himself described as a "libertarian theory of jurisprudence," and tried to pass that off as "original intent."