Uhm, it is very related to the population. Not sure if you ever tried to level with RDF on a realm where only a few % of people are still leveling and the rest are already maxed (or the realm population is overall low), but its normally quite difficult to find a RDF group at all since there are maybe 4 players total online in your level range of the same faction, and half of them isnt queuing. RDF wait-times are over 1hour and when it finally pops you will find 2/5 is afk. You can go on global and try to gather a group manually and then queue, but often there is no response either.
I aint saying they would enable RDF if that was the case (yes, they have other reasons too, mindset n stuff) but you may agree it makes a difference. On Lordaeron the intense leveling population would push 90% of players into abusing the instant-pop rdf to level with (there would be people spamming RDF button all day like lvl 80s used to do in Dalaran, because there would be enough players always in queue. On a "normal, average" realm this couldnt even happen because of lacking leveling population).
And that is something the staff wants to avoid for sure.

