Here's why:
* Shakespearean sonnets have a specific rhyme scheme: ABAB CDCD EFEF GG. The final two lines, known as the couplet, are a rhyming pair (GG).
* Petrarchan sonnets have a different structure and rhyme scheme. They consist of an octave (eight lines) with the rhyme scheme ABBAABBA and a sestet (six lines) with a variety of rhyme schemes, but not a couplet at the end.
Therefore, the heroic couplet, which is a pair of rhyming lines in iambic pentameter, is a characteristic feature of Shakespearean sonnets, not Petrarchan sonnets.