Throwing stuff out is hard to do if you are poor or were raised poor. Everything you save now does not have to be bought later. There is a potential use for anything and everything, so you might as well keep it. Granted, at a certain point the cost of the storage facility exceeds the amount saved, but the mindset is still there.

Even now, as well off as I am, I tend to collect things (and by extension, clutter) because I can see the potential need for something in the future. I've been trying to overcome this mindset and slim down my possessions to the things I need and the few sentimental items I have, but it's not easy at all. If someone else came into my house and started trying to clean up, I'd probably lose my mind (admittedly for a few unrelated reasons as well, but largely because of this mindset)