Because adultery requires that a person first be married.
And since virtually no one is married as proved by the fact that virtually every marriage can be annulled, it then follows that even less have committed adultery, because not everyone who is married commits adultery.
And of those virtually non existent few who have committed adultery, even less have died in that state of sin, because not all who commit adultery die in that state.
And so, because marriage is the the most rare of all the sacraments where even holy orders occurs much more frequently as proven by annulments granted to virtually all who apply.
And because the grounds applied by marriage tribunals virtually precludes adultery since adultery is grounds for annulment, i.e. unable to form a bond, it then follows that No one, or virtually no one, is in hell for committing adultery.
Now some may say I'm simply being cynical, but truth be told I'm looking at the bright side of the annulment scandal because adultery will send a man to a deeper level of hell than will fornication.