Health insurance marketplaces need to have set times in which people can enroll in order to maintain an accurate pool of healthy members! If people could apply any time during the year, they would only sign up when they got sick and then drop the plan when they got better. The stability of the market relies on healthy people contributing all year round to maintain protections for everyone.
This is why open enrollment exists! Everyone applies for next years’ plans during a set period of time and then everyone starts on 1/1/2020.
The only way a plan could start sooner is during Special Enrollment which requires a qualifying event. Follow the link for more information!