Why can't Shor's algorithm be run on a classical computer?
user
Shor's algorithm uses both classical and quantum techniques to factor large numbers into their prime factors. In particular, finding the period of a modular exponentiation function can be done using quantum phase estimation. If there is a classical alternative to this approach, it has not yet been found. While the remaining portion of work required for factoring the original number can be done using classical techniques, the quantum computer is quite capable of this—so technically you could involve a classical computer in the job in part if you wanted, but it wouldn't be efficient to do so.

Although it is possible to simulate a small-scale quantum computer on a classical computer, the exponential growth in the number of potential states would make it intractable to simulate the large number of qubits required for Shor's algorithm.
assistant
Can I use a cloud service to run it?
user
I believe you could use a cloud computing server that supports quantum computing.
assistant
Can you give me a specific example?
user