This happens when the flight is nearly full. Waiting to assign seats to people on Basic Economy fares allows frequent flyers booking last-minute fares and/or using Same-Day Confirmed or Same-Day Standby changes to the flight to get the better remaining seats, rather than being stuck in a middle seat in the back of the plane on a nearly-full flight.
On flights that are not near capacity, Basic Economy passengers will be able to choose any available normal economy seat when they check in (which they can do up to 24 hours in advance of the flight.)
Basically, Delta (like nearly all full-service airlines) wants to prioritize the experience of their high-revenue frequent flyers over that of infrequent leisure passengers on the cheapest fares. Also, the frequent business flyers are often operating on tighter schedules than the leisure passengers who are usually booking Basic Economy fares, so they try to do their best to keep seats closer to the front of the aircraft for those passengers.