I think it comes down to Resto Druid being top tier in every single category when you think about what they bring to a M+ group, the only slight weakness would be spot healing but even that one can be decently covered with the proper talent build, Druid also has a very flexible talent tree where a lot of things can work in M+ depending on what you need for the dungeon (other healers have less options in their builds).
Lets see:
HPS
The only healer that arguably can have more throughput in M+ is Monk, but even then Resto Druid is more mana efficient and has access to stronger CDs.
DPS
In general just by applying sunfire to the pack of mobs you can do a decent contribution, or if the group is good at avoiding damage you can spec into Feral affinity and do great DPS through catweaving (even better than Disc priest if given the chance).
Defensives
Resto Druid definitely took a nerf in this category compared to Legion, but having on-demand access to Bear form plus Barkskin in such a short CD is still very good. Depending on the situation I’d rank Resto Druid somewhere between No1 and No3 on this category.
Mobility
(no need to explain)
Utility
I think Druids, Shamans and Monks are the 3 healers with the strongest dungeon utility, this again depends on the situation, so dungeons and affixes play a part in what’s the most useful utility.
The most valuable tool any healer brings is probably Brez, every group always wants to have one and Druid is the only healer that brings it, the nerf to Blood DK also makes Brez even more valuable.
Druid is also one of only two classes that can remove enrages and having stealth on a healer is great too: Not only it enables death skips but also if there’s a wipe after skipping mobs you can run back in stealth and rez your party.
Ease of play
I’d say that mastering a Resto Druid can be pretty difficult, there’s a lot of room for min/maxing, specially if you take your DPS into account and try to catweave for extra damage. However playing Resto Druid it at a base level to be effective at healing without trying to min/max everything is quite simple, most groups are ok with you just focusing on healing and keeping them alife.