Suzy Khimm investigates. One reason? American women are more scared of divorce:

[I]f divorce rates are higher in a society, women have a higher incentive to obtain work experience in case they find themselves alone in the future. … European women anticipate not getting divorced as often and hence find less reason to insure themselves by working as much as American women.

Another factor:

Our divorce argument is further supported by empirical studies finding that US states that adopted "no-fault" divorce laws in the 1970s experienced a spike in female labour supply relative to other states (no-fault divorces do not require a showing of wrong-doing by either party). This is also precisely the time when Americans gradually started to work more than the Europeans. Most European countries adopted no-fault divorce laws later and still have significantly lower divorce rates. Our tax argument finds support in studies pointing out that European and American tax levels were about equal around 1970 when they began to diverge.

We conclude that in response to changing social norms, American women have moved to insure their own future by working harder, while European men in response to higher taxes have found it less attractive to work long hours.