Gatcha games aren't as lucrative on the NA side as they are in the SEA side of things. We spend peanuts compared to the amount of money our fellow gamers on the other side of the Pacific spend. I guess between the low financial gains and the cost for translation and server infrastructure just put the nail in the coffin for RK, sadly.
It doesn't help that more modern Gatcha games are better equipped for new players and have better retention and Gatcha rates usually. I've personally shifted my attention mostly from RK to Arknights at this point as I feel I can make better progression in AK versus RK for the same amount of time.