Opera Buffa was designed for the common people with common problems. High language was avoided for lower class dialog.
Opera Buffa or comedic opera started in Italy. It was designed for the common person with common problems wherein these problems were shown in a comedic way, so that the common person could understand it and also find it funny.
Opera Seria is an opera that was based on mythical subjects such as gods or ancient heroes. This type of opera could have some comedic scenes, but it was predominantly serious unlike Opera Buffa that would have mostly comedic scenes, characters, and entire plots with comedy in contemporary settings. Opera Seria, like Opera Buffa had three acts though some Opera Buffa only had two acts.
- Opera Buffa
Opera Buffa or comedic opera started in Italy. It was designed for the common person with common problems wherein these problems were shown in a comedic way, so that the common person could understand it and also find it funny.
- Opera Seria
Opera Seria is an opera that was based on mythical subjects such as gods or ancient heroes. This type of opera could have some comedic scenes, but it was predominantly serious unlike Opera Buffa that would have mostly comedic scenes, characters, and entire plots with comedy in contemporary settings. Opera Seria, like Opera Buffa had three acts though some Opera Buffa only had two acts.
- Historical Figures
Opera Buffa usually involved servants and therefore it had less to do with historical figures being depicted in the operas. As it has been mentioned Opera Buffa was for the regular people, so if there were historical figures it was minimal such as how these figures may have interacted with the regular people. For the most part there may have been references to historical figures of the times in context of a comedic line, but mostly those depicted were everyday normal. Opera Seria on the other hand was made for kings and nobility; therefore, it was an opera with historical figures such as kings and other nobility.
The question being asked is therefore incorrect. It is not Opera Buffa that had the mythical and historical figures, but Opera Seria. Opera Seria did develop at the same time or nearly the same time as Opera Buffa. In fact according to history it was a parallel development.