I'm no expert, but the halt USED to be part of lower level tests when I started eventing.
I believe the thinking was to encourage the horse to go forward, which is an important part of learning how to event...thus they took out the halt. This year is the first time since then that you have to halt on entrance even at Prelim...and previously, at prelim, the halt was well into the test when forward, one hopes, was firmly established.
I guess they got tired of seeing novice horses halt, and then trot off without any of the impulsion they had managed to develop prior to turning down center line.
While this is certainly a flaw in straight dressage, it can actually be dangerous, later in the day, in eventing.