That is because there are a lot of obstacles that need to be dealt with before an untethered iOS jailbreak could be possible. For starters, the hack requires an exploitable vulnerability in the system to be discovered. But after each jailbreak, Apple quickly patches its system, seals the hole, thus rendering jailbreaking the latest iOS version impossible, until a new flaw in the system is found. And tracking them takes time, which is why it took 10 months until a way to hack iDevices with the A5 chip was discovered.
So the more advanced and more hack-proof iOS becomes, the harder it will be to enjoy the untethered jailbreak experience on it, and all the goodies it brings along. That said, if you absolutely insist on having a jailbroken iPhone, better hold on to an older model before upgrading to an iPhone 5.
source: Information Week via TUAW