Garland is too old. They want a younger person who can be there for 30 years or more. Breyer is retiring now so that Biden can appoint a more hard-left replacement—after the November 2022 election the Democrats may be in a minority in the Senate and Biden or Kamala would have to nominate a less extreme candidate to get confirmation.
which is exactly why he is leaving now, he knows Republicans will be in charge of senate after November
Age will play into the selection...as well...it probably has to be a black female standing judge.