People wanted him in, plus I think they were working to reimplement him in the game by giving him a new cyborg robot design so that his return could make sense in DOA6
he was already an en existing character and is a move stealer so he was probably one of the easier one to bring back, cause from what I remember deadlines and budgetary concerns explain how we got certain characters (Ein and Leon got sold as DLC because they couldn't make them in time for the launch roster, Bankotsubo straight up couldn't make it in because of deadlines, and Nyo was cheaper to make)
It was basically a case of people wanting a character in because they were missing, then barely anyone using him like expected. Just like Lei Wulong in T7.
unpopular or not I still am very grateful and happy that Raidou was in DOA5 and will be in DOA6. He’s a badass villain, I don’t get why don’t like him, he’s not the easiest character to use but he’s really fun.