In all fairness, Obama's use of a smartphone was unprecedented. I read (but cannot find a reasonable source now) that Obama's Blackberry was tethered to a private base station, not any kind of public network, cellular or otherwise. So "secure smartphone" is a term that really means "secure infrastructure".
Supposedly his phone is also locked down to just ~10 other similarly secure numbers, while Clinton wanted to be able to use her secure phone to call her entire staff. That would've required dozens or hundreds more devices, which is a problem when every possible loss or compromise of any device is such a huge deal.
That infrastructure simply isn't scaleable.
[Edit: See this HN comment for sources. https://news.ycombinator.com/item?id=11306380]