Combination of factors, and yes, eventually. Its low value/high weight, so not a priority for shipping but is usually carted on trains, whose opoeration was greatly curtailed during the pandemic's early days. Due to a host of non-pandemic reasons, many common oyster-fishing areas are shut down, and others voluntarily shut down at the start of the pandemic, because their was insufficient restaraunt/grocery demand to make them profitable to operate.
There's also a building boom (fading), and some surprising uses for oyster shell in some locations. Its used for making "green" concrete as a partial substitute for two of the ingredients - burnt oyster shell makes like for the cement, while crushed oyster shell can be used ass the aggregate as well. Look up "Tabby".
Finally, its popular (or was popular) crushed as a driveway in many coastal towns. House prices appreciating as they are, it was (still is in places) more cost effective to use truckloads of the stuff to refresh an old driveway on a house you were seekign to sell while retaining rustic local character than it was to strip, re grade, form, and =pour concrete or asphault. Also much faster.
But yes, my local fishmonger is reopend, 40 miles away, and the shells are piling up behind the building. My local materials yards are both making piles of the stuff where they had empty ground, the railroads are mostly back, and it should trickle down to the pet food places - eventually.