Monday doesn't seem meaningfully later than Biden's resignation just three days earlier in 2009. If anything, it was Biden who broke precedent in 2009 by resigning almost two weeks later than the previous latest.
I think Biden specifically waited so he could be sworn in for his seventh term, and give his farewell speech as the seven-term senator from Delaware.
I'm not sure why Harris waited in contrast - especially given there is to be no farewell speech. Maybe there were lingering concerns she might be called back to vote on election-related matters?