why was it called -the hideaway ?

Yes, it was. At least that's what they keep saying in the book. I haven't seen this movie in years but I remember it being quite a disappointment.

"You idiot, that's condensation. It's on the outside!"

In the scene where he is taking the girl from the bar to see his "Art" she asks where they are going and he says "my hideaway".

The abandoned Wonderland amusement park where Vassago kept his victims. That was his Hideaway. Hence the name of the book and movie. Hideaway.