Because it involves an FTL restart. And FTL would re-import the history when it would still be in the database. The net effect would be that everything looks to be still there.
Because this was originally meant more in a "I want to hide some things happened recently" way. Very much like browsers offer you to delete the data only partially (without loosing everything). I can see that there may be a need to get both. However, flushing the entirety of the logs can easily be obtained by deleting the database. Flushing it only partially is a bit more involved as you need an SQL instruction.