A nationwide buildout isn't easy. I would bet their buildout is going faster than T-Mo or AT&Ts. Since the Network Vision project started in October 2011 they have rebuilt almost 18,000 sites according to S4GRU data. Almost 10,000 of those have LTE, again according to S4GRU data. As I understand NV, it takes a physical rebuild of the site with antenna changes and hardware changes at each site. Future upgrades should be much simpler, possible as easy as pushing software upgrades to each site.
So, rather than ask why it's taking so long, I'm impressed that they have been rebuilding over 200 cell sites a week for the last 18 months
(I hope I didn't break any rules sharing data here)