For my sake, Star Wars MU*s suffer from a few blights (Same as Star Trek for that matter).
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People want to impact the Universe, but people get upset when the Universe doesn't match a canon version. So... which one is it? Do you want to play out the movies or do you want to deviate and create a new world... and in doing such lose some of that familiar canon?
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It is a Setting, not a Theme. Star Wars is a Universe to be within and within that Universe there is a lot that can be done theme wise. The underdog Rebels, the police state Empire, the trader trying to make his way, the bounty Hunter looking to become famous... so many different themes in there and often they are at odds -- this is what leads to playerbase separation as well in a lot of cases. Some people love to play Rebels, some love to play Empire, and those two themes are not conducive together.
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Acquisition. People may say that gear/ships etc. don't matter and they don't to them. But to some people these 'items' are synonymous with the movies. The Lightsaber, the Millenium Falcon, the Custom DL-44 Blaster. Sure, not everyone is attracted by those, but some are attracted by those. This sense of acquisition leads to the next part of accompanying challenge is the 'how' of acquiring which leads to all sorts of issues (space, trade, econ, envy of what others have, etc etc.)
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System Issues arise as well since there is a bad trade off in a lot of cases between Force Users and the normal person. It becomes either a punitive on Force Users for being special or so advantageous that's all people want to be (assuming people are entering a winmore mentality). Along with that, Star Wars has a glut of special about characters by design. Jedi are 'specials' and thus people may hold them up as who they want to be or who they don't want in the game to keep the field even. Regardless the position taken, it creates a tension.
In the end, Star Wars just has a lot of design tensions built into it that, I believe, make it difficult for the setting to sustain a long running game of interest. There are certainly games that have existed for a long time but as someone mentioned they are BarRP. The world cannot change because if it does, it creates tension. If FU are limited or free for all, it creates tension. If there's gear (or not) it creates tension. And this isn't the good type of plot tension, but the design tension.