Mainly because (as Vitalik says) "the whole Ethereum network has the power of a 1999 cell phone."
Although Urbit (like Ethereum) is precisely defined without dependencies, Urbit is not a consensus computation platform. It's for processing your own data on your own (virtual or physical) machine.
Consensus computing is incredibly inefficient and should be used only where absolutely required. Where consensus is not absolutely required, computing should be localized under the user's control. People often forget to include this component in their designs of the decentralized future. But in fact, Ethereum needs something like Urbit and vice versa.
One way to think about the difference between Ethereum and Urbit: it's like the difference between a superconductor and a regular conductor. On the one hand, superconductors are qualitatively different and fundamentally more powerful. On the other hand, there are no superconductors in your iPhone.
Although Urbit (like Ethereum) is precisely defined without dependencies, Urbit is not a consensus computation platform. It's for processing your own data on your own (virtual or physical) machine.
Consensus computing is incredibly inefficient and should be used only where absolutely required. Where consensus is not absolutely required, computing should be localized under the user's control. People often forget to include this component in their designs of the decentralized future. But in fact, Ethereum needs something like Urbit and vice versa.
One way to think about the difference between Ethereum and Urbit: it's like the difference between a superconductor and a regular conductor. On the one hand, superconductors are qualitatively different and fundamentally more powerful. On the other hand, there are no superconductors in your iPhone.