you forgot an important one: tailpipe emissions control

What about a weighted average that accounts for each makers total sales of each vehicle in computing the actual fleet average. A real world fleet if you will. It would have to be done at the end of year. Does this exist.

Reasons:
(a) tougher crash safety laws (US carmakers are not allowed to assume passengers are wearing seatbelts, eliminating a critical system in crash safety design, leading directly to higher vehicle mass – not so in Europe)
(b) consumer preference for heavier vehicles due to greater presumed safety in a crash, especially if the other party is an SUV or pick-up
(c) tougher acoustic emissions laws (in Europe)
(d) consumer preference for larger engine displacement which are quieter at the typical load actually required, and rumble at lower frequencies which are perceived as a mark of quality
(e) consumer preference for vehicles with quiet interiors, requiring substantial acoustic damping material
(f) consumer demand for comfort and convenience systems (A/C, electric windows, electric seats, navigation systems, sunroof etc.)
(g) increased average height and girth of passengers
(h) consumer demand for increased cargo space

In short, as consumers became more affluent, they had more money available for transportation and also demanded more safety. Until 2002, fuel economy was more or less irrelevant for US consumers because the gasoline was so cheap. Even today, weight for weight, it costs less than bottled water there.

In addition, the average age of the vehicle fleet is going up due to improved product quality – that means carmakers only have one chance per decade per customer to sell a vehicle, so of course they try to sell you the most expensive one they can. That usually means larger, heavier and loaded with more features.