I would suggest that you use this to submit a Technical issues request about this inability to run applications that require .net inside the sandbox.

However, I would imaging it would be a massive task to have .net framework run inside the sandbox as it would also need to identify why .net version is required by the program and load it. I wouldn’t think they would want to have the sandbox running with .net before you actually want to run the sandbox and if there was a need for .net.