Part of the reason that a wrongful death civil suit can succeed when criminal charges either don’t stick or can’t be proven is that there are significant differences between the two types of cases and legal processes.
The legal “burden of proof” differs markedly. The “burden of proof” of guilt in criminal trials is “beyond a reasonable doubt,” whereas for civil cases seeking monetary compensation the “burden of proof” is whether the defendant’s probably was negligent and whether the negligence probably caused the death. This is hardly a trifling semantic distinction, but rather a very substantive difference. Also, in civil cases a defendant cannot “take the fifth” to avoid self-incrimination, and any refusal to answer a question can be taken into consideration by a jury during its deliberations.