why isn't lego updating the 9V train sets to the current system?
It is all a matter of cost versus demand. For tens of years LEGO had a separate train theme, with transformers, special metal rails, lights, switches and individual train cars and full sets. But the cost of maintaining and developing these components did not weigh up against the limited demand there was for these sets. Not many kids (90% of LEGO’s demographic) will want to splurge on all these sets (a single complete set plus some extra track is often enough for them), and the adult audience is loud, but not large enough to make such a complete line profitable.

That is why LEGO scaled back and just releases a few full train sets every few years, plus rails and switches made with a very simple design (involving only cheap plastic). AFOLs can and will design their own train cars/waggons anyway, so that is an avenue you can explore with your son.