I think (based on little to no actual evidence) that spammers ran out of money since "shitcoins on Bitcoin" has ran out of steam and the prior high fee environment forced everyone to be more efficient.

A year ago not even everyone had implemented segwit yet, now exchanges have become much more effective with batching and enabling lightning. Plebs have also come to terms with the fact that on chain is not for coffee and have moved to lightning or liquid.