OK, I'm gonna bite.
A RIS can be refused for reasons outside the requesting pilot's knowledge. eg
1) Poor radar performance in some areas. This can be because of such things as high ground, wind farms and the like.
2) ATCO workload. We do not possess infinite capacity. There is only a finite amount of time to monitor progress of flights, and to speak and receive transmissions and pass flight and traffic info.
Just for starters!
However, I would like to refer back to llanfairpg's point that it was likely that the "lower" a/c was climbing out from a strip, and therefore renders the whole radar point a bit irrelevant.
Thankfully, "mid-airs" are rare. Anything that we can learn from this will be valuable, I hope, to ALL of us.
Foxy