This whole article is an advert for this companies own new secure messenger because…

Why Would We Stop Using Signal?

We had a security breach of some root keys for a legacy chat server we were running and it got attacked and destroyed. It was too hard to restore after the attack and was abandoned. We tracked down the data leak to Signal, as the engineers had used Signal to send these keys between themselves.

Human error. Why are you allowing private keys on untrusted devices?