Wouldn't taking down the Empire have been easier with two Jedi? I'm looking for in-universe reasons and storytelling explanations.
The easiest explanation is that Leia was more valuable in her role as a diplomat for the rebellion. Being a Jedi was basically a death sentence after Order 66 and Kenobi either didn't want to risk both children of Anakin at the same time by exposing them as Jedi or didn't find it feasible.
It's strategically sound. Less risk, more reward. Leia has been very active in the rebellion by the time Luke picks up his first lightsaber. And if something happens to luke, Kenobi and Yoda seem confident they can try to train Leia in the force if they need.