[scikaiju]

Because more than likely Raiga is still a little kid at his point and time in the Garo timeline, therefore would not be Garo yet Kouga would still be. Considering how close they became despite the rocky start Kouga did ask Rei to train his son when he turned 10. And seeing how tight knit this group became I'm sure one of them would have asked at least once "Have you heard anything about Kouga and Karou yet?"

Also in the episode where Rei started training Raiga he told Gonza he was going to go away for a while but they would be back together, implying he was going to look for Kouga and Karou as well. Since both of them are still gone when Makai no Hana started, more than likely Rei is still gone too. So at most Dragon Blood took place maybe five years or so before he started training Raiga