@skinnylatte I would venture to guess that the Yucatán peninsula has spicier food because habanero chiles grow easily there and have been part of the local cuisine since they were introduced by the Spanish.
The serrano chile is from the central mountainous areas of Puebla and Hidalgo so central Mexico cuisine and has a decent amount of spice level. The habanero being a lot spicier than the serrano of course.
The northeastern states seem to have more US influence and less spice, while the northwest these days is a melting pot of cultures from different Mexican states so you find a variety of spice levels.