DISCLAIMER
First let me mention that these here are MY opinions and thoughts about RDF being disabled, you don't have to agree, like or see merit in them. Do I dislike RDF, not necessarily but I do see merit in it not being enabled. At times with 10k players on Lordaeron it's safe to assume RDF being disabled isn't as off-putting as it is being made out to be.
END DISCLAIMER
At one time or another "most" players have utilized the RDF system for whatever reason(s). With the release of the Lordaeron "Hardcore" Realm, a push for community amongst players, to again traverse the lands we have all come to know and an overall quality experience was/is sought.
Without RDF character progression is based "purely" off of the efforts of the players themselves without the crutch of the rewards from using RDF, this goes along with the marketplace being limited/expensive, no transfers to the realm and it being "hardcore", etc. On Lordaeron ("especially") players are putting in effort for their characters and RDF being disabled bolsters this by not providing an "easy street" to gold, XP and or emblems. Weekly raids "will" be even more appealing.
How many times have players queued for RDF only to have the group disbanded because players don't like what dungeon was selected due to things like "it's boring" or "too long" (especially at max level)? How many times have groups had to re-queue because someone didn't click "enter dungeon"? Without RDF players have to form their own groups and manually go to the dungeon(s) of their choosing which means interacting with players instead of being piled together based on the RDF system and not coming across a dungeon they wouldn't want to complete. Without RDF there is no more waiting for the queue to pop and possibly having to re-queue again and again until you get a group that doesn't disband and or all members click "enter dungeon" (though forming groups may still take time but it isn't the same thing).
How many times have players been randomly grouped with someone who has/had malicious intentions (ninja looters, intentional group wipers, etc.)? Without RDF players become more responsible for their actions and conduct which can't easily be circumvented by queuing for RDF since it doesn't take player conduct into consideration when grouping. Groups that are unaware of the nature of such player(s) will be vulnerable to them. Without RDF the population can shun negative players by making others aware of their actions and not grouping with them which would make it harder for them to "sneak" into groups by using RDF. Even if players are known to be negative it will still be a burden for groups to have to replace them if encountered with RDF.