TLDR; Content cost developers time and money, when the number of players that can use or experience the content is limited the cost of the content increases. By making the content repeatable and experienceable by all the return on investment is maximized. Content that is used once or not often is seen as a waste of investment. Every Branch in story is looked at as a loss in investment.
Time and time again I read threads that say, "What I am looking for in an MMORPG is a Single Player Game." There are people who want mmo's to utilize single use quest and events. If a server has over it's lifetime of say 5 years 3,000 players only 1 player will get to accomplish that quest.
Imagine that a Fantasy game, where a Evil Overlord (EOL) resides in a massive keep guarded by hundreds of minions both beast and humanoid. The quest has multiple starting NPCs because several NPCs in the world know of this threat and all want it eliminated. There is the veteran in the Tavern who was part of a previous attempt on the keep, he tells a Warrior Player Character about the EOL. The Warrior quickly shares the quest with 5 Guild mates and off they go. An Old Rogue in the Theives Guild offers to sell what he knows about EOL to a Young Shadow Walker Player Character. And so begins the quest of the Stealth class.
Lets say when the quest goes live at 11 A.M. EST, 65 of the 88 players logged in out of the 1,237 players who have characters on the server on this date get and decide to do the quest. In small groups they make their way to the Keep of the EOL. This is an Open World game, with no instances. As all 65 players approach the Keep, the game realizing they are all on the same quest in the quest / Keep area auto invites them to a single massive raid group. The group is not forced because you have to accept to join it. Let's assume that a 54 player raid forms and 11 players go it solo.
As the peloton progress into the Keep, doors are closed, and bridges are burned. This cuts off anyone who revives back at the start of the Keep. No one is watching the backs of the solo players and 5 revive and are cut off from the quest. The raid losses 3 players this way before it learns to wait on Rez's after the fight dies down. Now a document is found it explains that the way is split into three paths, A, B, & C. Each path ends in a lock that will open the door for the next path but close the door for the current path. So A opens B but closes off A and Trapping those sent into A.
This game has a class with an ability to summon other players at the cost of a token. But the raid only has 2 players of this class and only 7 tokens. It is decided that the raid will divide into three groups, the C group will have the two summoners and they will summon the 7 agreed upon members from the other two groups at the final door. The 6 solo players stick to group C want to move forward on their own. Group A opens path B and closes path A. Group B opens path C and closes path B. Group C closed path C and opens the final path. Surprisedly this door opens to Group A. The summoners and solo players are cut off from continuing and the remaining must fast travel to their last bind point.
The 17 player characters in group A move forward into the keep. The group now has 5 Healers down from the 17 that started. The rest of the raid is made up of 3 Tank spec's and 9 DPS spec'ed. Long story short, the remaining players defeated the EOL boss who drops a Tank chest piece, All 17 players get the achievement for defeating the EOL. But now the EOL is gone from the world, and the three paths are closed for ever. The first 10 levels of the keep are still open and repeatable. The remains of the original elite minion forces still remain, but now have lower stats and drop lesser quality gear.
The result is a permanent effect on the game world but only a fraction of the games total and eventual population ever gets to experience the content. The return on investment for the development of the content is very low. Maybe 5 players are ever repeatedly there for the new content to experience or achieve the content. Everyone else gets nothing or gets to experience the lesser repeatable content. The game gets the reputation of being a niche game, if you don't take time off from work or school to get the content when it goes live you will lose out on it.
Maybe these problems could be fixed by making it a yearly one week event with the return of the EOL or his next incarnation.