Because it costs $1000/mo and insurance wants to make it as hard as possible to get that covered because they cannot afford to pay $1000/mo * 45% of Americans without doing things to their rates that are forbidden by the ACA and, for that matter, any approximation of good sense. If people cannot afford an additional $450/mo each on average for their health care, how do you cover a critical long-term $1000/mo drug that 45% of the population needs?

Gating it behind mandatory expensive, difficult-to-schedule appointments with a specialist who is in abruptly short supply where the insurance company is doing their damndest to kick as many of them off their network as they can without getting caught to keep the shortage going is certainly part of that strategy. And the result is “people do not stay on the drug”, which is their goal, and if they don’t meet that goal they have an even bigger problem and can’t continue to exist as a functioning company.