There are several reasons why Chernobyl is off-limits, but all of them have to do with the fact that the place is a hotbed for deadly, life-threatening, body-mutilating radiation. Now, if that doesn't make you want to steer away from this place as far as possible, then we can only recommend that you seek professional help or something that'll calm your death drive a little bit.
Chernobyl's nuclear disaster in 1986 is one of the most well-known and awe-inspiring manmade disasters of the 20th century, and with good reason. It practically turned an entire city into a ghost town.