what are the reasons for these FSO / Event only people not wanting to go into the MA? . . . What is wrong with MA that stops you wanting to fly in it?
Nothing is wrong with the MA. It is just as it needs to be. It is always available, and it is excellent for getting up quickly in whatever plane you want.
To me, the MA is far superior -- a whole higher dimension of play -- compared to a single-player flying game. Because you are fighting with and against other people, which is a far richer experience.
To me, Scenarios are another dimension above MA, though. Scenarios are the closest thing to flying in an environment like what you read about in 1st-hand accounts of WWII air battles. Fights involve multiple groups cooperating together, with historically themed resources and goals, and fights with 45 fighters and 50 bomber aircraft -- as large as many actual WWII missions.
Since Scenarios are special events that run only 2 times per year, if there is a Scenario running, I will be in that instead of anywhere else.
But I've been flying Aces High and Air Warrior before that for 38 years (since 1988). Up to about 2017, I flew in the MA a fair amount. Then, MA only to brush up on the airplane I'd be flying in the Scenario. And in the past 4 years, mainly just Scenarios. Scenarios are what I love best in on-line flying.