Why would macduff have been a better king directly after king duncan's death?
It depends on the meaning of the question.
If you mean would Macduff have made a better king generally than Macbeth, then a study of their characters in the play would reveal the answer. Macbeth's characteristics include bravery and courage but also treachery and murder. Macduff proves himself brave, honest and faithful as does his family, who are murdered on Macbeth's orders. Contrast that with Lady Macbeth's character.
If your question relates to the word "directly", then I don't think there is any significance other than what I have already stated. Shakespeare wrote Macbeth after James VI of Scotland had ascended to the English crown. James was descended from Duncan so within the play neither Macbeth or Macduff were suitable successors to Duncan. Only his own lineage, Malcolm or Donalbain could succeed legitimately and that is what happens.