In the US the first amendment protections given to free expression make defamation claims significantly harder to pursue than in some other countries. o-called 'product defamation" claims are generally harder yet.
In the case of Dominion Voting Systems some of those sued seem to have made fairly specific allegations, which, if true, would involve probably criminal wrong doing. And please note that none of those suits have yet had a trial on the merits, to the best of my knowledge. We don't know if the statements complained of will be held to be defamatory or protected.
Claims that a vaccine is not as safe as it should be, or the regulators were too quick to approve it, are harder to frame as defamatory of the drug companies. Since the government contracted in advance for enough vaccine to give a dose to everyone in the US (as I understand it) damages would be hard to prove.
And there would be a risk of a PR backlash.
It is not as if any of these companies has tried to file a suit and had it dismissed. They have not chosen to file, for which there could be many reasons.