Everyone else's stats is as high/higher than JC even with him having access to tier 8 gear. Main reason is that Luminara is a straight upgrade to JC and some people don't want to use both on the same team.
Because there are diminishing returns with everything and the gear difference isn't that much, and because JC is solid but unspectacular, and because people want to make different builds, and because of Patch 66 making Jedi a liability unless you have a very solid team built around them.
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He's a decent healer with low hp and decent damage, and he's a jedi. Pretty obvious you can do better even if he's an above average hero.
Everyone else's stats is as high/higher than JC even with him having access to tier 8 gear. Main reason is that Luminara is a straight upgrade to JC and some people don't want to use both on the same team.