Basically there are three reasons....and these reasons will be rather pointed.

1.  It lacks or has no curb appeal.  This sets the stage.  If there is nothing on the outside the buyers are already tainted and they see (assumption) nothing on the inside.  Number 1 disaster. 

2.  De-clutter.  Get rid of pictures, furniture, whatever.  Be a minimalist.  Buyers want to see in their minds what their furnishings and pictures will look like in the house, not yours.  And don't put it in the garage.  Purchase a storage unit and store the items there.  Add the cost of storage to the price.

3.  It stinks (ok I will be nicer...it smells).  Cigarette smoke and animal odors....these are killers.    You have to take steps to make your home smell clean.  There are a number of products on the market that help with this or you can hire professionals.  Add this cost to the list price.

That's it....short, sweet and to the point.  Oh...one more thing to consider.....you may be priced too high compared to others in the general area.  If it is have a list of things you've done to the house within the last year, two years, etc. to demonstrate why it's priced as is.