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 😉

Theres low liquidity during the Asian session after the Asian session ends, it serves as a guide for determining what the next session (London session) is going to do in terms of price action. To find out more please read this :

https://www.mytradingland.com/thread/how-to-trade-eurusd-with-the-breakout-strategy-7391d8

The Asian is partially bad for trading because of low liquidity and volatility leading to narrow, range bound price consolidation rather than clear trends.

The Asian session is mainly dominated by comoanies doing business in/with China, Japan, Australia, etc. During the Asian session what goes on is mainly a bidding process so price action can be unpredictable. However, after the Asian session ends, you can mark it out on your chat and use its highest and lowest points as a reference to trade the London session.