It is because Network Vision involves tearing down every piece of equipment and every cable and cabinet at every site. It is not just adding LTE. Sprint is replacing its entire network and replacing with new state of the art multi-modal sites. This takes a lot of time per site and there is a huge learning curve when they get started.
And Sprint is starting the training and working with crews in less dense population areas on the outskirts and then working in.This result in a better and overall faster deployment. If they started in dense areas, any up front issues get magnified over a large customer base in a small area. Makes more sense to start in fringe rural areas.
There are a lot less sites in the outlier areas, so they are wrapping up fast and production is ramping up as the move inward. Progress will be rapid. Also, the FCC would probably take exception to your ideas for deployment. Focusing on wealthy and urban customers first is likely a violation of their rules.
Robert