I would guess that after reducing the original qualification thresholds, and giving Amex cardholders the $2,500 head start with no minimum spend requirement, they thought also giving MQD credit for Delta Stays would make qualification too easy.

Undoubtedly, as this is now the third time this promo has taken place this year, they are analyzing the metrics and likely considering making this a permanent thing... or will just continue to have promo periods next year.

I will say, the RDMs they give out for Delta Stays are a pittance compared to the commission Expedia is getting, so whatever cut Delta is getting from Expedia is probably a great deal for them. For large chain hotels, Expedia commissions run around 15%. (In fact, on a Hilton stay booked through Delta Stays this summer, I was able to pull up my receipt in my account post-stay and confirmed that the commission was a bit under 14%). For independent hotels, the commission is much higher.