Leia never agreed to be trained. Luke was just told to pass on what he had learned. Obviously the assumption was that would be to Leia, but if Leia had different plans - like starting a family with Han - it’d make sense that he might hold off, knowing he could train Ben (and others).

And again, we don’t know that Luke didn’t teach Leia the ways of the Force, just that she didn’t become a Jedi (we know at least that she has the power to sense her husband dying on another planet). There’s a lot about this topic we don’t know yet.