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As of the High Republic era there were 3 Grand Masters on the Council- Yoda, Lahru, and Pra-tre Veter. But by the PT era there was only one Grand Master- Yoda.

Was it ever explained why the PT Jedi never chose to elect more than one Grand Master again?

Nope. Not yet, anyway. I wouldn't be surprised if it's addressed in Phase III.

Probably a case of too many chefs in the same kitchen. Having 3 GMs all calling the shots can muddy the waters. Especially if they aren't all in agreement on a certain subject- like if Lahru wants you to do one thing, but Yoda wants you to do something else, who do you side with?

Having one GM who is ultimately deferred to is potentially a lot less complicated, and streamlines things significantly.