Because oil can stimulate your gall bladder. If you have food in your gall bladder it might make you feel bad. It's best to stimulate on an empty stomach when the gall bladder has built up pressure. Then it's like a mini liver flush.
But other's have pulled on a full stomach with no apparent distress. It's just more efficient not to eat first.
The reason you do it first thing in the morning is to kill the bad bacteria that have been festering in your mouth all night. Also the liver stimulant.....