SSB fixed with location not defined due to implementation point for BWP.
We have larger bandwidth moving to the center frequency via - 100khz raster [LTE] is challenging
Here 20 RBs are fixed location may vary as per NR-ARFCN-Band / GSCN based on RAN OEM implementations which makes the search quite easier for SSB.
Different SCS allowed for the SSB is also one of the major reason for BWP as we can settle SSB in any different location.
Note: whereas we have NCD-SSB also which you can read here may be all conclusive answer will give you details👇🏻