Because it was on the nightstand next to my bed for months, and I would read parts of it when I felt like doing that. With all that goes on when moving out of state, I forgot to put in back in the car before it went on the transporter, so it went in a box when packing up my house.
Anyway, I reset the FSS per these instructions posted by forum member "Spensley" (thank you, Spensley), so all is back to normal now:

RESETTING THE FSS MAINTENANCE INDICATOR AFTER AN OIL CHANGE

1. Move the ignition switch to the “RUN” position.
2. Within 4 seconds of moving the ignition switch into the
“RUN” position, press the reset button for the trip odometer
twice within one second. The display for the FSS service
request is activated for 10 seconds
3. During the 10 seconds the FSS service request is displayed,
move the ignition switch to the “OFF” position.
The FSS service request will continue to be displayed.
4. Press the reset button for the trip odometer and
hold.
5. Move the ignition switch to the “RUN” position
6. Continue to press and hold the reset button for the trip
odometer (about 10 seconds) until the audible signal
sounds and the new start distance appears in the display.
The tool symbol should disappear at this time.