You do not have a floppy driver installed.
Slackware handles installing drivers differently from other distributions. Most distributions install every driver possible and then load the ones that you actually need with initrd. Slackware probes all of your hardware during install and only installs drivers for hardware that you actually have. So my guess is that your floppy was unplugged or powered off when you installed Slackware and so Slackware did not install floppy.o
The two ways to fix this problem are to reinstall Slackware with all hardware turned on or to recompile your kernel with floppy support enabled.
Be prepared. Create a LifeBoat CD.