Just another day in paradise
I wouldn't say I'm so knowledgable about it, but to answer the question:
1. Rugby was amateur until circa '95 while league had been professional for 100 years or so. It's quite widely known that league defensive systems in the 90s were superior to union. It's why league defensive coaches were employed in union in the late 90s/early 2000s and is a very large part of why union became so boring.
2. I played rugby in the 80s, rugby & league (mainly league) in the 90s, and went back to mainly union in the late 90s/early 2000s. I had some pretty good coaches in both codes, and played with and against some very good players (who played for both the All Blacks and the Kiwis). Note: I was an average player (maybe worse), I am not claiming to be good at either code, but we were coached in completely different tackle techniques in terms of body positioning.
In union, we learned as kids to tackle low and bring the player down so you could form a ruck or get the turnover. In union you didn't want them staying upright in the tackle (most of the time) as it can lead to a rolling maul which is a pain in the arse to defend against (legalised obstruction and shepherding). In league, we were taught to wrap up the ball before bringing the player down or you leave a hole if they offload after contact. When I went back to union I had to readjust my technique to go lower as union forwards to tended to run with lower body positions than league (where forwards tended to run more upright).
I couldn't tell you specifically why the differences came about, it could be amateur vs professionalism. It could have been due to league utilising gang tackling earlier.
I'd also say that union tackling has started to look more like league tackling in the last 15-20 years (albeit without the wrestling/grappling).
Some of the better experts here could tell you more (there are some members who played at a lot higher levels of each code than me).
I dont want to brag about my rugby prowess, given I only got as far as club rugby U21's 😞 , but your answer is on point.
Also had a go at rugby league, but found it a completely different beast to Union.