This article from SFMTA's site gives other reasons for the bus substitution, including performing Muni Metro tunnel work and streetcar maintenance.
https://www.sfmta.com/blog/starting-...ervice-changes
Hopefully, MUNI will also use the time to place real seating on the already-in-service Siemens cars which somehow came with a sorry excuse for seating: too-high, too-hard benches.