Why are sweep and mend exclusive operations?

Mend is used to prepare a broken database for backup. As it is broken, there is a high chance that sweep would fail - it's enough gfix needs to deal with valid records, why should it bother with outdated ones. In any case, if you have a corrupted database and are using mend to fix it, it doesn't make much sense to clear up the broken database file from old record versions. If you really want it, you can try to run sweep separately.

Please note that mend is not normally required - only when you have a corrupt database. In such cases, you need to mend, backup and if all goes well, restore that backup. The newly restored database would not have any garbage anyway, so there's no need for sweep at all.