There’s just not enough information available for anyone to have a reliable predictor of what will make someone become a serial. There are definitely the common things among serials, but they aren’t always present, and aren’t present in every serial. You know, the whole animal torture/killing thing, escalating lesser crimes, etc.
So it’s impossible to know why more serials are men since we don’t even know why anyone goes serial in the first place. There’s not even definitive proof that the percentage of women serial killing represents all of them, or just the ones that got caught. For all we know, the numbers could be the same, and women are just better at getting away with it.
So, there’s not really an answer to the question, though there are theories.
The two theories I ran across that were the most convincing are that “triggering” whatever it is that makes people become serials is more likely to occur with men; and that there may be some linkage to the differences in hormones and brain development that go back to the womb. There’s also the idea that it’s both of those in conjunction, which seems more likely that it being only one of them.