IIRC, the reason is that the Nitro engine compiles directly to native code, which is then executed. This requires writing to memory as data and then executing the data. However, Apple doesn’t allow third-party applications to execute data for any reason. This would violate security policies. Thus, any and all third-party scripting—be it JavaScript in a Webkit view or something else—must be interpreted. The best a third-party application can do is compile to byte codes and then interpret the byte codes.
IIRC, the reason is that the Nitro engine compiles directly to native code, which is then executed. This requires writing to memory as data and then executing the data. However, Apple doesn’t allow third-party applications to execute data for any reason. This would violate security policies. Thus, any and all third-party scripting—be it JavaScript in a Webkit view or something else—must be interpreted. The best a third-party application can do is compile to byte codes and then interpret the byte codes.