Why does paapa go to the husband if a wife commits paapa after marriage? Why does it not go to the wife?
It is incorrect to say that the fruit of sin goes only to the husband after marriage. Sins and merits are shared equally between husband and wife, for as per the Manusmriti, Vedic rites have been enjoined for them jointly.
The Brihaspati smriti explains that husband and wife are like two halves and the fruit of action is shared by those two halves. Here the emphasis is more on the wife getting the fruit of the husband’s paapa and punya. Relevant extract is as follows:
A wife is considered half the body (of her husband), equally sharing the result of his good or wicked deeds; whether she ascends the pile after him, or chooses to survive him leading a virtuous life, she promotes the welfare of her husband.
-Brihaspati Smriti Chapter XXIV verse 11
For explicit mention of the husband bearing the fruits of the wife’s actions you can have a look at: Are husband and wife, associated/affected by each others' sins?
So to conclude, after marriage, the sins of the husband too go to the wife and vice versa. It is not that only the husband is affected by the wife’s paapa and punya.