Kids' TV's total abandonment of battle shonen is pretty interesting. Nicktoons practically turned into the Dragonball Z Kai channel in the early 2010s but once they finished a rerun of GT, they dropped the franchise quickly. Dragonball Super was created specifically to appease Western kids' channels by taking out almost all the blood that ironically made it a hit with the kids. Despite the franchise being inexorably linked with Cartoon Network in the 00s, Cartoon Network noped on Super. Dragonball Daima is probably getting on Adult Swim despite looking kiddier than Super. Don't know why Cartoon Network and Nicktoons turned on the Dragonball franchise.
I would say the main advantage of Kai is the fact that most of the filler was taken out, which makes it easier to get into the show compared to the original version. The Saiyan Saga going from 35 episodes to 18 is huge for example.
As for the blood, part of that is because Kai and Super aired in the mornings, where content is much stricter in Japan compared to prime time, which is where the original DBZ aired. Another example of this is Code Geass where the first season aired late night and the second season aired early afternoon.
Rurouni Kenshin is very much a historical Japan setting, which, again, may have been confusing or off-putting to a lot of American viewers, however good the characters or action may have been.
I feel like in Kenshin's case, going from weekday Toonami to Saturday nights hurt it. Generally, feudal Japan does pretty well with American audiences as we can see with the recent Shogun remake being a critical and commercial success.