These people were, to my knowledge, formerly known as "roommates," in virtually all contexts.
Housemate seems to be merely one of the many trendy terms being tossed around these days.
Here are possible reasons for this word's existence:
A) Perhaps it's supposed to have less of the negative stigma that the word "roommate" seems to carry with it.
B) Also possible is that roommate implies a closer relationship/friendship than simply housemate.
That guy across the hall, he's my housemate.