Cause its broken!! ..... Or maybe your low on fuel!
No, really, the only thing I can think off is a short of some sort. I don't know much about this stuff (So I probably shouldn't even be posting lol), but maybe something that is on all the time is grounding off into it. I had the same problem with an electric fuel pump. It was grounding off into the right side turn signal indicator!
I had a similar problem whit my wife’s Red-69 XR7.
I had just replaced some cables from battery and mounted a “ Head switch ” for the ground cable. Then when a put everything back a accidentally placed one cable at the wrong side of the starter solenoid so the LOW FUEL kept on lightning what ever I do. Then a found out that the connecting should be at the side facing the firewall ( starter terminal ) instead of the side facing the battery. I think When you engage the starter the LOW FUEL shall light for testing the lamp
Hope you fix it