There can be many answers for this.  Let me give you two possibilities:

• There is more divorce now because women have more economic options than they once did.  In the old days, women did not typically have the means to make a good living for themselves.  Therefore, once married, they had to pretty much stay with their husbands no matter what since they had no way to support themselves.
• Second, you can argue that there has been a decrease in Americans' willingness to sacrifice.  You can argue that Americans used to be willing to forego complete happiness so as to keep their marriages together.  Now, they are more likely to think that, if their marriage is not perfect, they should get out of it and seek their own happiness no matter what the impact on their families might be.