It got the green light when Razbam decided to enter into a developer relationship with ED, they presented ED a list of various aircraft as part of their initial developer agreement and it is on the work-bench and had been for quite some time as one of Razbam's projects... If you google Razbam DCS F-15E you will find some old WIP renders and videos of the F-15E project.
I think the F-15E was put on hold until the development of ED's air to ground radar which should be coming soon with the F/A-18C. Same happened to the A-6 Intruder too, most likely.