In his comedies, Shakespeare very typically devices a plot using two couples that intertwine, instead of just one couple. One of the couples usually represents the the comic elements of the play while the other character represents the morals found in the play. In Much Ado About Nothing, Hero and Claudio represent the moral element and therefore are the more important characters. Benedick and Beatrice, on the other hand, represent the comic elements of the play and are therefore less important.
One of the morals we see represented through Hero and Claudio is that people can easily be deceived. We not only see this when Claudio is easily duped by Don John into believing that Hero is unchaste, we see it when we realize that Hero is really the only true "hero" of the story. While both Claudio and Don Pedro appear to be honorable men, due to their ease at being deceived, we can actually question the uprightness of their characters. While Claudio and Don Pedro may be considered to possess the bravery of heroes, having just returned from a successful war, if they allow themselves to be easily duped, then we cannot say that they possess the virtuous mind and moral strength of heroes. Don Pedro even instinctively disbelieved his brother, as we see when Don Pedro says in reply to his brother's accusation against Hero, "I will not think it" (III.ii.100). However, despite his natural hesitation, Don Pedro still allowed himself to be tricked by someone he already knew had questionable character, calling into question the strength of his own character.
Instead, Hero is the one who remains heroic throughout the play. She bears Claudio's disgraceful accusations without growing angry at Claudio, only questioning his sanity, as we see in her line, "Is my lord well that he doth speak so wide?" (IV.i.59). She also very bravely stands her ground and continues to proclaim her innocence. She also agrees to follow the friar's advice and very bravely goes into hiding to fake her own death with the hopes that, "This wedding day / Perhaps is but prolonged" (IV.i.263-264). She also completely forgives Claudio enough to marry him at the end of the play regardless of his extremely poor actions. Not only that, Hero is also the embodiment of virtue. She truly is the loving, modest, chaste, virtuous maid she proclaims to be. All of these things show us Shakespeare's moral lesson that people are easily deceived, showing us exactly why Hero's story is what is central to the play rather than Beatrice's.
In contrast, Beatrice and Benedick serve as the comic relief. Through their battle of wits they both exchange very humorous lines. Also, despite what they both say against love and marriage, and also, despite that they both say they hate each other, they are also both easily duped into falling in love with each other, which also adds a comic element to the story line.
Hence, we see that Hero's and Claudio's story line is the more serious and important story line because it relays the moral while Benedick's and Beatrice's story line is only for comic effect, making it the subplot, rather than Hero's and Claudio's story line.