Because Meng Jingxian can put down her body. Or rather, she has no bottom line.
There are two famous modern poems in Kitajima, which may be his masterpieces: "Meanness is the pass of the mean, and nobility is the epitaph of the noble." Yes, mean, or there is no bottom line, or it is enough to put down the body, and in many cases, it can be passed unimpeded.
Huanbi married the king of Guojun, and also used some means. However, Huanbi is a very conflicted person, struggling between pride and low self-esteem. On the one hand, she wants to use Zhen Huan to realize her ideal of "entering the ancestral hall of the Zhen family" by recognizing her ancestors and returning to her biological mother, and on the other hand, she is secretly comparing and even competing with Zhen Huan, and there is a delusion of "if I bring you down, I can replace you" - when she was in the harem, she briefly betrayed Zhen Huan and formed an alliance with Cao Guiren, which is the manifestation of this idea.
After marrying King Guojun, Huan Biming knew that she was a substitute for Zhen Huan, and she couldn't help but have a sense of satisfaction of "knowing that you love him, but I married him, and knowing that you love her, but you married me". This kind of satisfaction supported her and made her willing to be the intermediary between King Guojun and Zhen Huan, and this satisfaction also restrained her, so that she could not take the initiative to get close to King Guojun.
Everyone knows that King Guojun loves Zhen Huan. So once Huanbi and King Guojun have a husband and wife, everyone will think that it is Huanbi who "seduces" King Guojun. Huanbi refused to let others tell the truth.
Meng Jingxian is different. Although she was born in the Guogong Mansion, she let out the wind early in the morning, and she was thirsty for the king of Guojun and became ill with love, that is, she kidnapped the king of Guojun with this love, and then married the king of Guojun. It's so ruined, I really don't even want to lose face. It's hard for her father, holding the title of the duke, messing around with her daughter, and losing face.
Meng Jingxian is also a smart person. She knows King Guojun's character of pity and cherishing jade: even if she doesn't love it, she can't bear to hurt it. So she pretended to be delicate everywhere, which caused King Guojun to take care of her as a responsibility, so she became closer and closer.
King Guojun didn't love Meng Jingxian. Meng Jingxian also knew this, and had no unrealistic illusions, thinking that "the stone can also cover the heat", and sooner or later she will influence the king of Guojun. She just got drunk with King Guojun and took the opportunity to conceive King Guojun's child.
King Guojun is also not disgusted with Huanbi. Even because of Zhen Huan's relationship, he has a more common language for Huanbi. If Huanbi also designed the drama of "getting drunk", she may not be able to get pregnant. It's just that as I said earlier, Huanbi's peculiar self-esteem restrained her, so that she couldn't easily come up with such a means as "seduction".
King Guojun only loves Zhen Huan and does not love other women; Huanbi has self-esteem and refuses to bow down. The vacuum between them was seized by Meng Jingxian, who had no bottom line.