The simple answer is because Microsoft disallows you from modifying hardware that you own. This is a very real power. I personally do not have the capability to produce my own hardware.
The more complicated answer is that the big guns (Microsoft, Google, Apple, etc) have manipulated the government into allowing them these powers. So now we don't just have a technological problem, but a social problem. When we have an entire society of people running to the store asking them to put the glossy digital handcuffs on them, we encourage that kind of behavior. We make it economically viable for a corporation to oppress a large number of people. Circling back to my original point, this makes it difficult or impossible for someone to hack on a new hardware platform that's been locked down.
Imagine if there were a ban on the color green. Artists would be completely deprived of a large part of their pallette, and their art would suffer for it. Tivoized hardware hurts hackers in exactly the same way.
This is the first example that came to mind, there are definitely plenty of others. Basically it boils down to societal problems that manifest themselves as technology.