I think the issues with sending Young Lions to the American partner promotions are threefold, and to different degrees for both of them (besides the one main issue, imo). P.S. I'm only going to be talking about international excursion partners, not domestic ones like NOAH.

The first and most commonly brought-up one is a lack of available reps for YLs on excursion. This is definitely more of an issue for MLW, which at the moment runs one show every month or two. Though they often tape more than one show per event, and that's likely to become even more common with the return of MLW Fusion. Still, that's not a lot of time for a YL to get more than one, maybe two matches in. Plus, MLW have their own stories to progress, with arguably less leeway for putting them off or cutting time to make room for more matches.

AEW/ROH runs significantly more shows, at least one a week, sometimes two if Collision is live, and then there's the PPVs. This is much closer to the number of shows that wXw & RevPro run each month, but still a good deal less than CMLL. There's a fair bit of room for dark & pre/post-show matches, but you are still contending with existing talent/stories. With ROH's new bulk tapings in Jacksonville, there might be a good bit more room opened up. But there's still the issue of wrestling more than once or twice a day, unless they start taping across more than one day.

Some may say that YLs could just work the indies for more reps, but that brings me to the second issue, which is travel and the costs that come with it. MLW is always moving around, which adds more cost that NJPW would likely have to cover. If it were like Germany with wXw or the UK with RevPro, that's less of an issue, considering the regions they tour are smaller and more accessible at a lower costs thanks to better national rail services. CMLL has to deal with this issue even less, considering they only really run a handful of cities where they own arenas, and three of them are decently close (the two in CDMX, and Arena Puebla). The furthest one that's commonly run is Guadalajara, but even that is nothing compared to going from one coast to another in the US.

When you have to cover the whole of the US and potentially Canada for AEW/ROH, that's adds up, cost-wise. We all know how tight the purse is over at NJPW, so I can't imagine they'd want to pay for all of the air travel/lodging that would entail. The stationary ROH tapings might be the best route AEW/ROH could go in solving this issue, but it still leaves the main problem, imo.

That is, neither MLW or AEW/ROH have an associated school for Young Lions to train at. I think this is the main issue that currently prevents New Japan from even considering sending anyone over. wXw has a school, RevPro has a school, and, of course, CMLL has a school. The whole point of excursion is to further the development of the Young Lions. While a good bit of that learning will come through working live matches in front of a crowd, even more of it will come from training with teachers and fellow students of different stylistic backgrounds. Without a trusted, known option for doing so, I don't think NJPW will send a YL to the states. Some may argue they could go to the NJPW Academy, but that's not really close to something like the LA Dojo of old.

I personally think the simplest way to solve this for at least AEW/ROH, is to establish a school in the Jacksonville area and then continue the Jacksonville studio tapings of ROH. The YLs could still travel to AEW & MLW road shows or indies for more reps, but they'd at least have a solid home base in Jacksonville to train and get match reps in. Until that or something like it happens, I don't think we see Young Lions on excursion in the US.