Guys, come on...the most obvious reason is that the IAAF is f*cking the distance events on the diamond league. No more 5,000m or 10,000m races included as "diamond" events. Obviously the 10,000 has been out for a long time, and has been reduced to relative obscurity.
But, the big reason is obvious, and that is that the longer events on the track are receiving reduced funding. The road races will always have large crowds by default due to the participation of the masses. I am not pretending to be an omnipotent expert on this matter, but speaking from this on a purely logical angle.