I'm not quite sure why we're talking about star wars in a thread about expanding horizons, as there are plenty of better places to do that.
However...
The primary reason why Disney star wars/Disney Marvel/etc is doing poorly is because of the movement to make politically correct media. It's fine to have minority/female characters, but they work too hard at trying to make them perfect and flawless (AKA, the biggest writing sin ever). Rey is a hard core mary sue. Anikin had to actually learn how to use the force, and Rey just beats someone who was personally taught by luke with zero training. She also just "finds" Luke's lightsaber in a random pub. Not "A" lightsaber, LUKES lightsaber! Anyway, I could rant about this for hours, but other people have done this for me, in less time.
It happened to come up, and I think if you really want to crack this nut open, the focus should be the new verse, not the old dead one.
Disney is killing it, with the exception of the failed Solo film. They're learning slowly that you can Make new films and you can get by, but if you want to dig up beloved canon and characters, doing irrelevant cash-cow titles, they will fail. This is the most iconic science fiction material in the world. You don't toss its trophy characters around in cheap.
It's actually becoming an issue, because Disney now knows they can pump out any type of generic, watered down, family-friendly, conceptually safe crap, and it will turn out millions in profits.
You need look no further than the MCU for all the proof you need. I don't think the MCU has let it's audience truly feel an emotional or dark moment, until 'The Snap'. There's always a joke wedged in, to make sure you're smiling and laughing from open to close. They meticulously attack every angle of these films to ensure mass appeal.
Man of Steel had more character development than all of the MCU combined, I.W. notwithstanding. Yet people go into MCU films primed to adore it, and go into DC movies primed to hate everything about them.
Disney is breast feeding the industry powdered milk right now. They meticulously craft universally lovable films that anyone from a 7 year old rookie, to the 56 year old grandpa can fully digest in a single sitting, without thinking, or feeling too much, because in today's hyper-sensitive society, feelings carry risk.
In terms of your PW specifically,
Expanding Horizons. I don't know what happened to it, but in twelve years of RP, on many, many different sites, I can you tell what happens to a lot of PWs, and it's far less specific than you might be thinking. These things die on a regular basis and have been since the dawn of play-by-post.
First of all you have to understand that a lot of people on average get excited by the romantic notions of PW, sign up, then given the proper time, realize it's probably too much work and effort. We see this just in RPs, let alone something o the scale of a PW.
Most PWs die due to lack of actual direction and organization. You can take all the time and care in the world to design the settings, the lore, create all the factions and primary locations, work out the running plots and sub-plots, etc. But if there is no PW team to manage and guide those plots, and keep the player base organized, informed, and 'in the loop' so to speak, it will lose any and all individual interest. PWs are great, but there has to be a real point... Sandboxes are doomed to fail, because it's essentially chaos: A petri dish for characters.
I had seen someone else say that staff created it and then more or less abandoned it. To me this is hearsay because I don't know, but if there's even a grain of truth to that, you needn't look any further for answers. PWs don't look after themselves. Stories need a certain level of structure to be good, and therefore rewarding for serious players.