USC is popular for a lot of reasons, but by far, I think the reason it is most popular is because it is closer to home. USC is THE pharmacy school for SoCal residents. Considering the number of students both above and south of LA who are pre-pharmacy and USC's large class size, it is no wonder that USC is the priority choice for most students. Speaking to graduates, they don't nearly have such enthusiasm. They usually make (income-wise) no more than their competing pharmacists and resent their larger debt. However, for these people, they didn't have the luxury to go ANYWHERE. Also, for most CA residents, leaving CA is not really a desired option.
The alumni association is a very popular reason. I don't think any USC member would increase a salary because of it, but in today's economy. AN edge is better than NONE, and it does exist. There's a lot of professional solidarity and pride with Trojans.
I personally do not think the concept of a time-honored curriculum or school to be a critical factor in a desire to go to a school (unless you are comparing Touro or Western with USC. However, UCSD has a relatively new curriculum and its graduates are highly desired as well.) I think what's important to consider is funding into the program. USC has a large nest egg as do the UCs compared to other schools. This gives room for opportunities, expansion, and development that some schools are just not able to do.