Why?
IRCC explained the request in fingerprinting, because they need to do background check on your application timeline (which was 2 years ago), then they receive clearance that during those period was everything okay.
Having requesting NEW fingerprint clearance a year later, as citizenship eligibility period was ALREADY cleared - just has juridically zero sense.
Its because IRCC can't work with an expired criminality clearance. Like I already mentioned, criminality clearance is valid only for 1 year. At the time of decision making, apparently, criminality, security and immigration clearances all have to be valid.
Because you were required to submit fingerprints before to obtain a criminality clearance, IRCC wants fingerprints again to get a new criminality clearance since the old one expired. RCMP, AFAIK doesn't store fingerprints. I think fingerprints are destroyed soon after a criminality clearance reported is issued.
As to "why" they have this policy in place, you'll have to ask IRCC about that.