Why is Auschwitz the most improbable tourist destination? A beer factory is visited by millions (Guiness) every year, yet we are surprised a place where millions of people were slaughtered is a popular tourist site?
Indeed, in the 70s-80s most visitors were survivors, families of victims, school and group trips. But with the advent of mass air travel the significant increase in visitors is not surprising. 
I grew up less than 600km (but 2 countries) away from Auschwitz. Started to read about Holocaust, WWII, concentration camps and the plight of the European Jewry when I was a kid. Always wanted to visit, but never managed until after I moved to Canada.
Yes, some buildings are not authentic (Germans blew up the crematoria before they left). Yet, if you step in, even knowing you are in a replica, it still churns your soul and heart.