Whoever owned the house and property, I don't know why they let the house fall into ruins and not
restore it. Just think, if that house was standing today, it would attract tourists who are fans of DS.
Speaking as someone who lives and works in the general vicinity of Lyndhurst, I would say the answer to that is money. That whole stretch along the Hudson river USED to be lined with millionaire's properties of that nature, but the cost of keeping the properties up, expecially once the Depression hit, and later, pretty much doomed most of those properties. Real pity. The only ones still surviving are either National Trust, Hudson River Trust, or private owners who use the properties for rentals for special events. Even if you go up to Lockwood matthews in Norwalk, Connecticut (HODS finale was shot there) you can see that its taking them ages to bring the house back to what it was in its prime. It, too was nearly lost before funding was acquired.
As to the fire that destroyed Spratt house, local urban legend, which I've seen repeated in DS fandom, is that it was squatters and cigarettes. And the property is far enough off the beaten path and was probably old and dry enough that the fire department couldn't save it.
Jeannie