3) Too expensive. 4) Takes too long.
Getting a patent attorney to prepare and file a patent is very expensive. It takes $10k just to get started. The patent system is so byzantine that you can't reasonably expect to do it yourself and be successful. The cost is a drop in the bucket to a large corporation, but it's a major barrier for individual inventors.
Not only that, but it takes way, way to long to actually receive a patent. I applied for a patent about two and a half years ago, and I'm still waiting.
As an individual without infinite resources, I'm hesitant to built a business around my concept unless I can be confident that it'll be protected. If I'm not granted the patent, then it would be trivial for an organization with deeper pockets replicate what I would have created.
I know that patents typically don't make a business defensible, but in my case I decided to pursue a different business opportunity. If I'm granted my patent, then I suppose I'll figure it what to do with it then. On the other hand, what good is a patent on a technology conceived three years ago, except to make life difficult for others?