There's a different (heightened) standard of proof for claiming defamation in the US too. Trump has good, dirty lawyers and investigators, and a defamation case brought by the Clintons would harm their reputation more than help it (or their bank account). It'll never happen.
I wouldn’t. Their lawyers would probably have a stab at it, if there was available evidence.
Snarky remarks aside, the Standard of proof required to demonstrate a loss in the USA is different to that used in England from what I have read.