1) There are a limited amount of buses running for early ADR's.
. . . most ADR's start at 8:05am
. . . a few buses, as mesaboy2 said, leave the barn at 6:30am
. . . they start arriving at resorts about 6:45am-65:50am
. . . there are not many, just enough to service the eatery patrons
2) The large flow of buses start approx 1½-hr before park hours.
. . . this is the WDW rush hour
. . . they start to arrive at resorts about 1-hr before gate time
. . . however, there are no buses from DTD to any parks
. . . this halts people from parkng for free, then grabbing a bus