There is no way that we can know for certain why a crime like stalking is on the rise. We cannot conduct experiments to determine what factor or factors leads to this. We can, however, speculate. Here are two possibilities.
First, it may be that stalking has not risen but that it is reported more often when it does happen. In other words, it may just be that people did not report stalking to the police in the past but do now. This could have come about because of greater awareness of the issue. Women may now feel more empowered to report stalkers when they might previously have thought that there was no point in doing so. This is particularly plausible since there have only been stalking laws for a relatively short time.
Second, it may be that men are acting more aggressively towards women. This could be happening because men feel that they are no longer in control of life in the US as they once were. Men are getting less education than women today. Men are more likely to be unemployed. It may be that men are feeling more resentful towards women and are wanting to act aggressively towards them.
However, there is no real way to know if either or both of these is actually the case.