I believe Minorin went over why she didn't tell him before hand. Takasu simply wouldn't believe her if he told her, because Taiga's dad comes off as such a nice guy to everyone else. He's exactly like Ami used to be. But he only forces Taiga to do things when he's alone with her. Forcing her to move all her shit out of the apartment, promising this, promising that, taking her to expensive restaurants without asking her where she'd actually like to go or even what she actually likes to eat, the list goes on and on. From the outside looking in, it looks like he really cares for Taiga, but you can't know that isn't true until he betrays your trust.Originally Posted by KrayZ33
Minorin didn't tell him outright because she fell for exactly the same routine. Unless he experienced his own viewpoint betrayed the same way Minorin's was and how Taiga always ends up, he simply wouldn't believe her.
For Takasu, it was even worse because he never had a father, and the only anecdotes he ever hears about his own father was what a badass Yakuza punk he was. Takasu views his father as a dirtbag, and without ever experiencing what a father was actually like (Yacchan only tells him that he was super cool, and she's pretty unreliable) Takasu was even further tricked into believing that Taiga's father was a good guy.
Takasu impressed his own vision of what finally getting his father back would be like, and made it worse because from the outside looking in, Taiga's dad is a really stand up guy, who like Takasu inwardly hopes about his own father, maybe just got a bad reputation undeservedly.