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).
For a time but sometimes sacrifices need to be made. Leia made such a sacrifice in canon and as a result Snoke was found an opening to worm his way in.
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.
I could have sworn I heard somewhere he gave her a beginners course in how to use The Force. Think Luke’s training in SW.