The question of PCT and CDT vs. AT thru hiker differences is an interesting one.
My first question would be: do we actually know how much less popular Osprey packs are on the two western US scenic trails than on the AT (if they are, in the first place)? I don't think we do. I certainly haven't seen any hard data supporting this.
But, assuming that it is true (and from my own limited anecdotal experience, it at least seems to be true), then I think a few things are going on…
1. There are certainly fewer people attempting the PCT and the CDT each year than the AT, and I think there is a slight selection bias going here as well, above and beyond the raw numbers. From that I mean that the average PCT or CDT thru hiker is slightly better informed in terms of gear selection than the average AT hiker (at least in my opinion). I think, "better informed" thru-hikers also generally trend toward lighter gear (and, hence, away from Osprey and more toward cottage manufacturers like ULA, Six Moons Designs, Gossamer Gear, Hyperlite Mountain Gear, etc…).
There are lots of people who attempt the AT and have absolutely no idea what they are getting into (just listen to stories of some of the crazy stuff people drop off at the outfitter 30 miles into the AT). I think this is at least somewhat less true on the PCT, and certainly much less true on the CDT.
2. I think that people who have already done one thru hike are also more likely to be on the PCT and CDT than the AT (though I have no data for this). These veteran thru hikers usually trend more toward lightweight gear as well (and, in following, away from traditional gear like Osprey's)
3. The PCT is basically an ideal proving ground for ultralight backpacking, with extended periods of dry, bug-free weather, and gently graded trail. This lends itself to lighter gear, and away from more traditional packs like those of Osprey. I think most CDT thru hikers have done the PCT (or portions of it), so they are most likely already "drinking the lightweight kool-aid" as well, even though the CDT doesn't have quite as ideal of conditions for UL gear choices as the PCT.
4. Many of the cottage gear companies who make the lighter weight backpacking gear are located in the western US. I think it is no coincidence that PCT and CDT hikers use more of this gear. Distributions networks and word of mouth count for a lot here, particularly for smaller companies. People forget that Osprey packs used to primarily be a western thing as well until they grew large enough to become an REI staple brand and their Colorado roots became much less relevant in their sales and marketing strategy.
Those are all my purely subjective theories. Feel free to shoot holes in them.