Here are the reasons why.
- MSSCCI is dedicated to Visual SourceSafe and is not compatible with Subversion;
- MSSCCI is dedicated to Checkout-Edit-Checkin approach, but Subversion is built for Edit-Merge-Commit;
- MSSCCI doesn't provide atomic transactions;
- MSSCCI provider can't show status of folders;
- only one MSSCCI provider is available at this time. There are use cases where Subversion and Visual SourceSafe controlled projects are used together in one solution.