Boeing cut a bunch of corners on safety and oversight so the executives could have a few more million in their bank accounts and they rolled out a space capsule that leaks and doesn’t work well enough for NASA to trust it.

Also people died on their planes for the same reason and they allegedly killed a whistleblower who was warning everyone about it. So. Y’know. Sort of a long series of faceplants in the service of corporate greed.

The gist of it is that to reenter the atmosphere safely, you need to point the heat shield at the oncoming air. To do that, you need working reaction control thrusters.

Boeing Starliner capsule apparently has some kind of latent failure mode with its thrusters where they start degrading and can fail in a few hours of operation. On the way up, the spacecraft was in transit for around 26 hours, and recorded five thrusters disabled, of which four relit in subsequent testing. One thruster is apparently really dead. The capsule has about fifty thrusters, so being down five doesn't really compromise steering.

Ground testing of these thrusters was performed at White Sands, NM, and apparently NASA really did not like the results. But I haven't heard anything about what those tests found.

There's also a helium leak (active only when the RCS is warmed up), but it's not clear if this is a critical factor. Compressed helium is used to force the fuel to go out of the thrusters.

As far as I know, the actual estimated risk of failure for this capsule is still pretty low, but now believed to be high enough that they might as well not take the risk. (And remember these are test astronauts... They have a pretty high risk tolerance).

Also, even if the risk of one reentry is maybe low enough to try it, the White Sands and orbital data probably revealed a systemic design problem with the RCS that now precludes operational certification. Since neither Boeing nor NASA is willing to pay for a second test flight, this capsule is effectively dead as an ongoing space program. So why take any kind of risk on a reentry if you don't have to?