We can make this argument on at least two grounds.
First, we can make it on the grounds of fairness. People with preexisting conditions deserve to have the same access to health care as anyone else does. The whole idea of insurance is that it should spread the risk across a large number of people so that the people who end up healthier will subsidize those who are not as lucky. This idea is destroyed if those with preexisting conditions are excluded.
Second, we can make the argument on economic grounds. It is important for our economy for people to be able to move from job to job. If people with preexisting conditions cannot get new insurance, they will not be able to make such career moves. This is bad for our economy.
Thus, we can argue for this on both ethical and pragmatic grounds.