We don't actually know much about the rate of serial killings historically, or even at present really. What is most likely is that the phenomenon of serial killing became more well known during the mid 20th century and began to be more closely investigated and tracked by law enforcement. Many serial killers pray on those who are at the fringes of civilized society such as prostitutes. To be quite honest, many police departments throughout history have been corrupt and lazy, and many of them would have put minimal effort into investigating the deaths of prostitutes or the indigent. Consider, for example, the Whitechapel "Jack the Ripper" murders. They garnered attention because they were so gruesome, unique, and came in quick succession (4 within a single month). But a series of killings of prostitutes in the same area by more conventional means at the rate of one or two a year would have blended into the background of violence in the poor neighborhoods of London and the police would have not bothered to pay attention. The same is likely true for homicides throughout the industrialized world up through the mid to late 20th century.

Additionally, in many countries national level police and investigators were much less active in investigating crimes of such a nature until around the 1970s or so. With the advent of the automobile in the early 20th century, some serial killers have taken advantage of the mobility provided by it in order to spread out their crimes and make them more difficult to investigate. Prior to the advent of good record keeping standards and ubiquitous telecommunications, a series of unexplained deaths spread across multiple municipalities were unlikely to be linked together before the late 20th century.

Even after serial killings became a subject of interest for police departments, it's still nearly impossible to determine the overall rate of serial killings with any degree of accuracy. With diminishing accuracy the farther back in time you go (even in the 20th century). This is because it can be difficult to tell if a particular homicide is part of a serial killing. Especially so if the overall rate of killing is not high or if no distinctive method of killing is employed. And the standards of investigation and collection of evidence have been considerably varied from municipality to municipality through the industrialized world over the past century and more. Even if a distinctive method of killing used in a homicide was noticed, say, a hundred years ago, that information may never have ended up being recorded in a file, there may never have even been a specific file for every homicide as well. These things make it difficult to determine the overall rates of serial killings because there is a sufficient number of "background" homicides year after year to hide a significant number of uncaught and unknown serial killers at any given time.

Consider the case of the Atlanta Ripper of 1911. Over the course of a single year, over 15 young black women in the Atlanta area were brutally murdered with their throats cut. This is a crime that is obvious, distinctive, and difficult to ignore. But if those black women had been murdered in a less distinctive fashion (or if evidence of the distinctiveness of their deaths had been lost by the time the bodies were found) and at a much lower rate, the Atlanta police at the time would certainly not have cared overly much.

Or, consider the case of H.H. Holmes, who built a "murder hotel" in 1888 and killed off dozens and perhaps hundreds over a period of only a few years. Had he been only slightly more careful he might never have been caught.

Moreover, the advent of popular national media through television and the ease of long-distance telecommunication made reporting of serial killers much easier from the mid 20th century onwards, and there was a growing fascination with the phenomenon that increased media coverage. This led to many serial killers becoming famous, household names from the 1970s onwards, irrespective of the overall rate of serial killings.

Add to that the increase in the importance of record keeping through the 20th century. With state issued identification cards becoming more common and every business at every level being forced to keep records more diligently due to tax laws and various regulations, it became a lot more difficult for people to simply disappear without notice and without a trace.

There's also an important effect that crime rates are relative to population size, and populations in the industrialized world increased greatly throughout the 20th century, which would be expected to lead to a corresponding increase in the number of serial killers.

Overall, it appears that the increase in known serial killers over the course of the 20th century is due more to an increase in detection and popularity than an increase in the propensity of individuals to become serial killers. This is likely due mostly to changes in policing, record keeping, and mass media.