why is the asian session bad for trading forex?
Because the risk does not equal the reward. You spend long hours trading the Asian session only to make a small profit because price is changing so slowly (they call this low volatility).
If it took you 2 hours to make $100 profit during the Asian session, it could take another person 30 minutes to make the same $100 profit during the London session, so tell me which session would you rather trade? I guess the London session 😉