Because women's names change with marriage, and will no longer match their original documents. As written, the act would discriminate against anyone who has had a name change, but the vast majority of those are married women.

In addition to what erin said, I also want to point out that changing your name does not automatically change your name in the vital records. You have to do that yourself and it's not that intuitive, plus it costs additional money on top of the other name change costs.

Being trans, I have experienced that myself and due to wording on the court order and from what the judge told to me, I incorrectly believed my birth record would automatically get updated. According to the vital record support person I talked to, that's actually a really common mistake and happens all the time.

I'm sure there are many people with outdated records simply because they too incorrectly believed that it would be an automatic thing. For some it's probably even intentional if they feel their married name doesn't belong on their birth record.