I've wondered about this question myself. The world is very big with lots of places to go that have much less risk than Mexico. I travel to many 3rd world countries on business that many people would not want to go to. So when I go on vacation I usually want to go to a place where security is not a significant concern. I get that most tourist spots in Mexico are statistically safe, and maybe safer than the risk of your random Walmart shooting in Laredo or downtown Houston.
But there are many violent instances that occur in Cancun, PV, PDC and other tourist spots that never hit the press. If you happen to be in the wrong place at the wrong time then the statistics don't matter, and the downside is not good. I'm not trying to be chicken little as I know most people go and have a great time, including me as I love Mexico. But I just don't see the need the take the extra risk when there are so many other places to go to. I expect some comments saying I am over exaggerating, but if you have seen the security reports I have in my line of work it might make you pause.