I think much of it has to do with SPF growing faster and being cheaper and less desirable where appearance and resistance to abuse is important.
The biggest problem I see building with cottonwood you milled yourself is a picky building inspector asking about the lack of grade stamps.
Labman, you're dead on with the first statement. Softwoods can grow to millable size in just a couple decades in the right environment (South-eastern US comes to mind) whereas it can take a couple centuries to grow a nice, big hardwood tree. Another upside is that softwoods can grow a couple hundred feet tall and be poker-straight for the entire length, whereas one is lucky to get more than a couple usable logs before a crotch in an average hardwood. Hardness is also an issue. Even Douglas Fir and Hemlock are hard enough to be undesirable for framing, let alone Oak or Maple. My house has some 50+ year old Douglas Fir joists in the basement, and if I hit a knot in it with the nailgun it just bounces off and leaves half the nail sticking out still. And doing it the old-fashioned way is next to impossible.
This is why there was such a huge logging boom in the PNW from the late 1800s onward - our easy access to these huge trees provided cheap, easy lumber to North America. A similar boom took place in Ontario in the mid-to-late 1800s which almost eradicated the old-growth White Pine there.
Re: using cottonwood for building - it's not even nearly as strong as Spruce-Pine-Fir for building. My lumber grading manual has a strength rating chart for all the common North American woods, and is very similar to an online one I found
HERE. Then actual numbers in mine are almost all different, some by a decent margin, but the general trend and differences between woods are very close. My chart has wet and dry numbers as well. At any rate the only structurally weaker woods than Cottonwood are the Cedars more properly from the Juniper family. On the other hand, Cottonwood/Aspen is very split resistant and can be planed to a glass-smooth finish, and can make a nice woodworking wood. My cousin has cottonwood/aspen T&G on the vaulted ceiling of his log house, it looks really nice.
I've often wondered about how an inspector would feel about my using self-milled wood in the house. I'm outside city limits so I can do work without permits at my own risk, but if/when I go to sell the house it'll still need to pass inspection. I am a AA ticketed graderman with 7 years' experience, so I'm not going to use crap wood in my own house, but I don't know if that would make a bit of difference to an inspector if he's wanting to see a grade stamp. But it hardly makes sense for me to pay the lumber association a membership fee to get a stamp number, so screw it.
