Actually, I don’t think it’s hard. A payment transactions can move any amount of money in fairly small amount of weight. Spam transactions generally take more weight per operation and most of them don’t have an open ended value to their senders.

With the waning interest in inscriptions and runes, feerates of spam transactions are minuscule:

(from this dashboard: https://dune.com/murchandamus/inscription-brc20-weight-and-percentage)

E.g., yesterday, spam transactions paid 4.7% of fees for 47% of the blockspace. Payment transactions paid 95.3% of fees for 53% of the blockspace. So, payment transactions are currently paying about 18× higher feerates on average than spam. To me that sounds like spam transactions are only bidding on underdemanded blockspace at the very bottom of the mempool around 0.1 s/vB. As the mempool is clearing out, they finally are selected into blocks after sitting there for an indeterminate amount of time. There just seems too little demand for blockspace from payment transactions right now.