Could well be, I'm sure there were lots of poisonings and whatnot that earlier forensic science misattributed to natural causes.

But it could also be that murderers are getting more savvy too. There have been so many CSI-style shows, police procedurals, and so forth that it doesn't take a great genius to know how to make it a lot harder for police to identify you. Like, if you made an untraceable "ghost gun" to shoot someone with, don't carry it around in your backpack for days after shooting the guy. Sheesh, basic stuff.