Mabye because even the d300s is marketed to consumers. Half the places I know that carry the d90 carry the D300s (like best buy), and the vast majority of D90 users aren't soccer moms but advanced enthuisasts, serious students, and art/humanitarian/low budjet pros. That is mainly because the control layout isn't quite as limited or crappy as the D5000 but cannot rival a slightly different layout few key features on the D300s body. Soccer moms dont buy the d90 because almost all of the "consumer" features are the same so why pay $400 more? It is the more professional features and layout that give the D90 it's edge.
Suprisingly the main users I find of the D300s are nature enthuisasts, press backups, minor league sports shooters, and middle class/rich consumers. Most "pros" in other fields either think that putting the extra $500 towards a D700 is worth it or cant afford the D300s due to the line of work they do.
The problem with the D90 is that in terms of usability it is obviously a class above the t2i/a550 but it comes short in a few key specs making it have a disadvantage to the 50D even though it outclasses it in many aspects...
The question is will the D95 kill off the D300s and D90 line in one shot and create a 7D killer or will it just make it go completely head to head with the 60D...
