Worked only 6 months and want to leave

...how do you explain to a potential employer why you are looking to leave and find a new position?

I think you can be honest and explain that the company was in turmoil, not doing well. Companies fold all the time.

Would it cost one if one speaks the blatant truth about uncertainty being the reason to leave?

It depends on how you frame it, particularly in behavioral rounds. One of the biggest purposes of that round is to prevent "Negative Nancies" from getting into the company. It's no fun having a teammate who is constantly complaining about everything (soooooo many engineers are like this). When you talk about this, you don't want to dwell on it for too long, and you should have a respectful, empathetic tone (e.g. say something like "Due to extreme market conditions, the company was constantly re-organizing itself, and that made it hard for any of the engineers to focus and deliver impact"). You don't want to get this point across as "The company did re-orgs, therefore they suck".

Zooming out, companies do re-orgs all the time, especially the bigger ones like FAANG. Every company is trying to reposition itself as an AI company, leading to further internal chaos. I would try to stay at the company longer if you can as a 6-month stint looks quite bad on your resume. Re-orgs really are just a part of life in tech.