Apart from the lack of sophistication regarding background saving, the two main reasons were
a) writing to disk was inherently dangerous: file systems of the time were not robust, so anything going wrong while updating metadata (in particular) could trash not just your file but the whole filesystem. Floppies were slow and machines were crashier, making the whole operation riskier; and
b) using floppies reduces their lifespan because the disk physically rubs against both the read/write heads and the internal padding of its casing, so typically they were only accessed as part of an explicit user request.
a) writing to disk was inherently dangerous: file systems of the time were not robust, so anything going wrong while updating metadata (in particular) could trash not just your file but the whole filesystem. Floppies were slow and machines were crashier, making the whole operation riskier; and
b) using floppies reduces their lifespan because the disk physically rubs against both the read/write heads and the internal padding of its casing, so typically they were only accessed as part of an explicit user request.