I have no doubt that it is likely to be marketing and profit motive primarily. On the other hand, I also think that the larger cassettes are better shifting. I've not done 10, but my new used bike has a 9 speed, where the old bike had 7. I definitely like the more subtle gear changes of the 9, and I think I'd like the 10 slightly better still. All that said, the 10 speed components are so expensive compared to 9, that I have no temptation to make hte change until it happens to occur on a future bike I buy.