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The other key distinction that might explain the rise in one and the decrease in the other is probability of success.  
It has become much harder to be a successful serial killer as technology and forensic science have improved alongside cameras (surveillance, ring cams, etc.) becoming practically ubiquitous.
Also the rise in mass shooters seems to roughly coincide with the decrease in serial killers. I think some a lot of it is the news covering mass shooters so much more than serial killers. 
While many say the psychology is not the same. People imitate what they know. How many would have taken the steps to go though with that form of violence if they hand been exposed to it so much.