Her father Joe Simpson's attempt to promote Jessica into stardom not by focusing on her singing, but having her gain as much weight as fast as possible. This crash diet led to Jessica gaining a tremendous amount of weight however her father did not also focus on strenghting Jessica's muscles and quickly Jessica became immobile and bedbound.
When her father Joe switched his promotional push to her sister Ashlee, Jessica attempted to lose some of her weight and also strengthen her muscles. Unfortunately for Jessica her efforts did not gain success and in depression she started to eat more and gain again.
Her mother Tina was able to gain custody of both Jessica and Ashlee, just before the younger Simpson became immobile, as Texas officials started to investigate her father for child endangerment as both Ashlee and her sister had been under 18 when they both become immobile.
When the Simpson sisters were interviewed in the early 2000s, Jessica's weight had passed 1000 pounds and by all reports she has been able to stop gaining but hasn't been able to lose any. Recent reports both Simpson sisters weighing in well over 1200 pounds, still immobile and bedbound in their family home in Richardson, Texas.