The CBO estimates is based on the availability through employers and even that isn't set in stone.
May article in the NYT:
One of the most notable features of the Senate proposals is that workers could drop out of an employer’s group health plan and buy private insurance on their own, outside the workplace. The employer’s normal contribution for a worker would be paid to the insurance exchange.
Democrats said that people dropping out of employer plans would, in many cases, be eligible for tax credits to defray their premiums.
Employers worry that this feature would destabilize the health plans they provide to employees.
“If people can opt out of employer-sponsored insurance and get a tax credit, that will lead to a death spiral for employer-sponsored plans,” said James P. Gelfand, senior manager of health policy at the United States Chamber of Commerce.
“People who are sick will stay in employer plans, and many young, healthy people will opt out,” Mr. Gelfand said.
linkThe CBO cannot predict how many people will opt out of their employer plan, which is nothing new.
On edit: The Chamber of Commerce guy evidently doesn't have a lot of faith in private insurance, despite the claim that people are happy with their insurance.
America’s Affordable Health Choices Act Section-by-Section Analysis