Two Reasons: First, it's a totally different technology than offset. The toner particles are manufactured to a specific particle size, shape, electrical charge, and heat temperature that is specific to your model of digital press. In offset, ink is ink. You can use a variety of inks from different manufacturers. But, the same is not true for toner. Using toner in your digital press that was not specifically formulated for that press will trash your engine.
Secondly, "Follow The Money". Unlike offset presses that can cost millions of dollars, digital press manufacturers run on a totally different business model. The machines themselves are relatively inexpensive (as compared to offset presses). They really don't make that much money selling or leasing equipment. The majority of their income comes from the toner and consumables. Not from the equipment itself. It's kind of like the small personal printer you have at home, only on a much larger scale. The printer itself may only cost $75, but, the ink cartridges that you need to buy to keep it running are usually anywhere from $35 to $50 each. That's where they make their money.
Case in point: (without getting in to exact specifics that may jeopardize our relationship with Xerox) - We're running (2) Xerox Versant 2100's. The price space of the V2100 is around $120,000 to $150,000. Our click charges are about average at around $0.05 per page. On the two machines, we average around 300,000 pages per month. Do the math, and, you will see we pay about $15,000 per month in click charges ($0.05 x 300,000 = $15,000). So, in one year, we will pay around $180,000 in clicks. The machines are on a 5-year lease, so, for the life of the lease, we will pay around $900,000 in clicks. That's close to One Million Dollars! As you can see, the original cost of the equipment pales in comparison to the amount of revenue they can generate from the click charges. They are not about to make it easy for a third party competitive entity to eat in to their profits.