..who asked: "I'd like to start with a question: why were the lower stories of the insulae more comfortable and expensive for the tenants?"

In addition to Cornellia's correct take on fire hazards the higher one dwelt, there were other benefits to living on the ground floor.

It was the only floor on which one could have running water. Despite Roman skill in hydraulics (and their understanding even of the concept of syphoning) I don't believe a domestic block has ever been found in which water was pumped above the ground floor.

In Roma itself, although there were regulations governing the diversion of waters from the acqueducts it is known that wealthy ground-floor dwellers did this.
If one could afford it, one could even plug one's apartment into the sewer system and install indoor latrines.

A final consideration is that on the 4th or 5th floor you could not construct a portico to shield your flat from the sun.. and those who have visited Rome in the months of high Summer know that this is something you SERIOUSLY want to shield yourself from!
Though I forget the reference, I recall it being claimed that top-floor dwellers in the summer could only really use their homes to sleep in, since they became uninhabitable once high noon kicked in.