Delta probably realized they weren't getting as much benefit from AMEX as they could, so significantly increased the spend requirement for DM.
The MQD waiver probably wasn't giving DL much benefit compared to the costs of elite status -- or so they think (hmmm)
The flyers who spend $250k on an AMEX or who spend $15k are the ones DL wants to "keep." This is the highest tier status after all. By thinning out the DM population, it makes the remaining DMs feel more valued with more upgrades, etc. Since these are the high-spenders, they will keep spending on DL based on their positive experiences.
This also seems to coincide with the desire for FCM. If more and more First Class seats are being sold for cash, it seems like DL would want the "top of the top" to get the upgrades first (as they unfortunately become more scare in the future).