Spectrum ownership and broadcast power limitations. To operate as a wireless service provider at the scale that cell phone companies do, you need the FCCs explicit permission. The process to get approval is long, expensive, and fraught with peril (see the LightSquared debacle [1]).
This kind of regulation is ultimately justifiable. You don't want service providers using conflicting technologies and hammering each others airspace. You quickly end up with what economists call the tragedy of the commons[2].
It's my impression that the fact that a fairly limited range of frequencies are allocated for mobile phone use is a barrier to entry.
I don't think that's the main difference between the US and say... Germany though. I'm pretty sure German carriers are required to sell each other access to their networks under reasonable and non-discriminatory terms, making it fairly easy to become a carrier without building a bunch of cell towers. The phones are all GSM and almost all unlocked, so it really is as easy to switch as getting a SIM from a new carrier.
This actually appears to be a case of more regulation, applied to just the right spot resulting in more competition. Politically, I'm not usually in favor of a lot of regulation, but it looks like it works in this case.
I believe because it is a government-granted monopoly there must be regulation to introduce competition or the customers will be taken advantage of (especially with something like internet which should be regulated as a utility in the first place).
Like your case in Germany, and elsewhere in Europe where they are required to lease the "last mile" at a reasonable price. Internet connections in countries with these regulations are much faster and the charges are less compared to the USA.
In the US, we HAD a history of controlling monopolies and oligopolies carefully - the trust-busting era, the steel breakup, the comparatively recent AT&T breakup - but the government has largely abdicated any significant role here. The notable exception was the proposed AT&T/T-Mobile merger, which, ironically, probably would have actually helped improve technology and push down prices because both of those carriers are moving into the prepaid/pay as you go space aggressively. Unfortunately, both also use GSM band technology...
With respect to unlocking, however, I can't fathom why the device manufacturers, particularly Google/Motorola Mobility, don't play the adults here. They have more to gain from impressing the end-consumer than making time with carriers. It has to be more expensive for manufacturers to create several different versions of the same device for different carriers. In many cases, these are all but different devices - not the same radios, not the same functions, often not even the same processors. If these manufacturers made their flagship devices with both CDMA and GSM radios operating on all common bands (deactivated or not, as need may be), they would benefit from economies of scale, lowering their costs and allowing the devices to sell, unlocked, at lower costs. This would make sales directly to end-consumers more realistic, which would FORCE the carriers to improve their service, in order to bring people into their no longer walled gardens. That would force improvements in speed, price, and quality.
Right now, I guess, manufacturers benefit from advertising done by the carriers, but they also suffer from the walled garden and loss of economies of scale. I have no way of knowing which is worth more to them, but we have yet to see a true, universal, unlocked device from a carrier. We can tweak world CDMA phones to "work" on GSM networks, but not too well so far.
