I don't know either, but considering the complete lack of existence of the kind of network SpaceX is proposing, or anything close to it, if it's not deployment costs it would have to be some other factor coupled with operating a ~1000 sat network.
They say it's due to lower orbits. Iridium is fewer satellites so they have to be higher. More w/ lower orbits means lower latency. Do you know if this is too good to be true?
40000 / pi / 2 = 6366 40000 / 2 * 3 = 60000 60000 / pi / 2 = 9550 9550 - 6366 = 3184
Per the above, any satellite Internet system below about 3000 km above the earth will have latency that's competitive with fiber optics on earth (note that I haven't done the simultaneous solution to account for the uplink/downlink distance so it's actually a bit less). For the SpaceX system, there should be better latency than the terrestrial Internet. Please ignore the rounding ... this is a back-of-the-envelope exercise.
Fiber optics are great and far less susceptible to interference than radio - but if you Google articles on how high-frequency trading companies are installing line-of-sight microwave for their networks you'll see that fiber isn't our fastest technology.