Wrath of the Lich King saw tons of players raiding. Raiding was considered more accessible than it had ever been before. For context: before Wrath, there was no such thing as a difficulty setting for raids. Burning Crusade introduced Heroic dungeons, but when it came to raids, there was only one version of each instance. Karazhan was a 10-player raid. Most of the BC raids were 25-player only.
And in terms of difficulty, raids were basically all either on heroic or mythic. Granted, that's my memory speaking. The truth is that if we were to revisit these raids we might actually consider them simpler than what we get now. But difficulty was achieved in different ways, like requiring certain raid compositions (for a time you had to have a Shaman in every DPS group so that each could benefit from Bloodlust/Heroism, as well as the various totems that they brought.) You also needed to farm potions (they used to simply have cooldowns, meaning that players would chug them over the course of a fight rather than use them at one specific point.) There was also resistance gear that certain boss fights needed.
Wrath launched with the revamped Naxxramas, which was considered a far easier raid than what had come before. While not all the raids were so easy (Ulduar was a big jump up in difficulty,) the ease of Naxxramas seemed to give players a real leg-up into raiding.
Complaints about how easy the raids had been in Wrath (most likely by a vocal minority) led to Blizzard greatly increasing the difficulty of raids (as well as dungeons) in Cataclysm. Raiding fell off significantly, with far fewer guilds making meaningful progress.
Blizzard felt that they wanted some method to ensure that players would be able to at least see the story's conclusion, and so they implemented LFR - a system built on the LFG tool that was introduced in the final patch of Wrath that would allow players to easily get into raids and fight easier versions of the bosses.
LFR made these epic encounters far more accessible. The difficulty of LFR has undergone some tuning over the expansions. Mists was the first expansion to have all of its tiers with LFR, and I would argue that LFR raids in Mists were actually more difficult that Wrath's version of Naxxramas. This meant that they were a challenge, yes, but also a slog when players did not have the patience to learn the mechanics. And a progression of dungeons like in Wrath and Cataclysm, there was no real option for alternate progression.
In Warlords, LFR's difficulty was significantly reduced, and it went on to serve its initial purpose quite well - allowing players without the time to spend doing "proper" raiding to see the raid environments. However, to prevent people from feeling they were forced to raid in order to complete tier sets and get powerful trinkets, the LFR rewards were made intentionally unappealing - trinkets had flat stats on them and there was an alternate tier set that had simpler bonuses. The appearances of the gear was also significantly drabber.
In Legion, we seem to have the best of both worlds - gear coming out of LFR is now once again just a less powerful version of what exists in the normal and higher difficulties, and they are just a re-color of that gear.
So why, then, am I not really running much LFR?
This may be a temporary problem - right now only one raid is open. The LFR gear that one gets out of it is 835, which is inferior to both Mythic dungeons (which is an admittedly small 5 item levels higher) and also the cap on world quest rewards, which is currently 845.
We're also not yet into raids that have tier sets - the gear out of Emerald Nightmare is just plain gear, with no real need to collect pieces to fit together.
Probably the biggest thing, though, is that we no longer have an expansion-long legendary chain. In the past, running LFR was a great way to ensure that you downed every boss and thus were able to maximize the quest items you could get out of a raid for these legendary chains.
What appears to be the equivalent to this: the "Balance of Power" quest chain given by Kalecgos, is only completable in Normal mode or above. So running LFR provides little benefit to players who have equivalent or better gear beyond getting together item appearances for transmog.
The thing is: is it more trouble than it's worth?
It's a hard question to answer, because honestly it's not a lot of trouble. While you'll get people who still don't know how to get out of a tree before Ilgynoth re-forms its eye, generally the fights are not that bad.
Emerald Nightmare is situated in a strange place on the power curve of the expansion, but I suspect that we might see LFR become a bigger deal as time goes on if things stay the way they are.
Right now, Mythic dungeons give better gear than LFR by a small amount. But in order to get beyond 840 gear (other than with lucky warforge/titanforge upgrades) you need to start running Mythic Plus. And while there are certainly plenty of people doing that, I suspect that the majority of players are not really interested in it. Once we start having LFR that rewards better gear than Mythic dungeons, I think that the system will become more attractive.
But then, do we really want to return to the dynamic of Mists and Warlords, where raids totally supplant the role of dungeons?
Obviously, Blizzard has already walked back the notion that new dungeons would have the same rewards as old ones, given that Return to Karazhan gives gear that I think is even better than Normal-mode Emerald Nightmare.
And in fact, the Dungeon Journal no longer even lists Raid Finder rewards (a bug I'm sure,) but using Atlas Loot to look up that gear, it appears that Trial of Valor and Nighthold LFR gear may actually be stuck at 835 - making it no better than Emerald Nightmare gear (except for the set bonuses in Nighthold.)
I don't really have the solution here. Ultimately I'm hoping to get past the issue by getting my guild back into regular Normal-mode raiding. I only hope that LFR can retain enough interested players to ensure that queue times don't get absurd.