They correspond to the larger eras in French economy.

• Industrial revolution
• Entre-deux-guerres, a period of strong urbanization and a huge push towards social housing. I suppose they included WW2 cause nothing was built there anyway.
• 1946 to 1970 is "les trente glorieuses", the time of rebuilding everything, which means everyone had a job and could afford a house or apartment.
• The oil crash in 1973 ushered in a more modern era, usually more left-wing after May 68 and with the election of Mitterrand in 1982.
• The 1990 one is around when we elected a right-wing president and the public policies vastly changed.
• 2005-2006 was starting to get tough because of oil again, I believe. It is also around the beginning of the US subprime crisis, of which the consequences affected us all too.