I think it’s two factors. First, they weren’t as prevalent as we think they were in the past, we just lump a half century’s worth of serial killer history into “recent past” and it feels like they were everywhere. Second I think it’s a lot harder to get away with now. The combination of better inter-agency communication in law enforcement, digitized fingerprint and dna databases and cameras everywhere mean that they get caught before they go on decade long multi-state killing spree.