I believe when talking about naval ships, commissioning is when they enter active service, so construction probably began early 90s, maybe even late 80s, and probably a few years of designing, bidding, etc before that. And of course there were probably all of the usual idiot politicians, bean counters, stubborn assholes, sales people, etc. involved who pushed for older tech. Maybe because everything else they had worked on the old disks, maybe they were skeptical of the new tech not being robust or tested enough or wouldn’t catch on, maybe it was just cheaper, etc.
I’m willing to bet that they somehow locked themselves into using 8 inch disks in the early to mid 80s if not earlier, when the 5¼ discs were still new-ish and the 3½ were brand-new or not even available yet.