I have a hypothesis (not tested by research yet) that one of the reasons the D.C. Circuit vacancies have ballooned to four, and three of them have dragged on for years, is that D.C. does not have voting Senators.
Other circuits have Senators for the states within them. Voters and others can question those Senators about what they are doing to move the process along. The Federal Circuit is also Senator-less. But its nominees face less opposition, since it is a specialized court and not typically viewed as a farm team for the Supreme Court, as the D.C. Circuit is.