The main reason for this is because JITting allows code-generation on-box, which potentially means that the code you're running on a client computer is different to the code that was audited by Microsoft during the on-boarding process for the appstore.
Consequently, Microsoft are looking to deprecate JITting for apps distributed through the appstore - at least initially for Windows RT, but later rolling out to all of Windows8.
This also has the benefit that the cloud can do really agressive optimisations, rather than having to perform a trade-off between optimising more and needing to start running the function as soon as possible.
It also has the benefit that it allows Microsoft to be consistent when it starts telling other developers that they're not supposed to use VirtualProtect/VirtualAlloc or build their own JITters in Metro apps when they're doing it themselves for .NET.
Finally it has the added benefit that cloud-JITting means that 100% of your battery power in .NET apps on WindowsRT are spent running code, rather than JITting code, which should stretch your battery life a little while longer.