There are at least three main reasons that could account for why this is happening.
First, there is the fact that women today are tending to be more educated than they once were. In the past, it was at least somewhat less likely for women to have college degrees. Today, women are at least as educated as men. This means that they are more likely to be qualified for well-paying jobs than they would have been in the past.
Second, there is the idea that economic competition will do away with wage discrimination. Economic theory suggests that it is impossible for firms to discriminate in the long term if they want to be successful. If firms discriminate against qualified women, they should, in the long term, lose out to their competitors. This may be driving down incidences of discrimination.
Finally, there are changes in the public perceptions of men and women. It is socially much less acceptable to hold (or at least to express) the attitude that women are in any way less capable than men. It is much more generally accepted that women can and should have careers. This allows women to become more educated and to participate in the workforce more. It also puts more pressure on firms to treat women equally.
These are three of the main possible reasons for this change.