It's not obvious why they banned 40 Watts but kept 32 Watts (which seems miniscule of a difference).
Googling, I found this: Why did US Department of Energy discontinued the T12 lights? http://www.t5fixtures.com/why-did-us-department-of-energy-discontinued-the-t12-lights/
Here is a direct quote of the main reason: "T12 light bulbs were becoming extremely inefficient"
Here is a second direct quote of the secondary reason: "Polychlorinaed Biphenyls are used in T12 fixture ballast manufacturing"
Huh? Why would a T12 ballast use PCBs while a T8 ballast wouldn't? Makes no sense to me. Does it make sense to you?
Googling some more, I find this: The Case Against T12 Bulbs that Invited the Ban
Which says (verbatim): "the conventional four-foot T12 lamp still consumes a whopping 40 watts every hour. On the other hand, the more modern T8 lamp consumes anything between 25 and 32 watts of energy in an hour."
So I guess the 8 watts mattered to the DOE.
SImilarly, it says "The T12 lamps are not long-lasting", but, since when does the DOE care about how long bulbs last (especially since incandescents don't last all that long either).
Now we get to the hazardous waste where it says "T12 lamps release toxic mercury and PCB waste products".
Huh? Why would T12s release more of these than T8s?