1) Download size
2) Speed
Especially in a country like Iran where bandwidth is limited, the download size of the Tor Browser alone is enough to make it used relatively rarely. Couple that with much slower speeds once you're up and running, and it's just a non-starter for many users.Beyond that, though, the vast majority of users are also simply not engaging in activity online that will realistically put them in danger. Users in censored regions are really like users anywhere else in that respect -- they are not doing illegal things online that would make them targets.
On the DPI front, every major censor definitely uses DPI in an effort to block tools like Lantern.
That said, Tor is great at what it does, and we strongly support the Tor team and their work and collaborate with them whenever we can.