It's not *that* low. League average is around 20% k rate and he was at 16% last year. He also was at 24.8% in 2013 back when his prospect status was still in tact. And he clearly has the stuff. A better guess is his command hasn't put him in position to take advantage of his weapons. His slider has a 25.8% k rate which isn't amazing but it's not bad either and it was 27.3% last year. If you look at his splits, his k rate on 0-2 is 39.0%. On 1-2 it's 31.9% and on 2-2 it's 29.9%. On 3-2 it drops to 17.2%.
To give some context, using Arrieta as an example on 0-2 he's 48.4%, 1-2 he's 47.3%, and 36.3% on 2-2 vs 21.4% at 3-2. Obviously Arrieta has an advantage based on these numbers from last year. When he was struggling a bit more in 2012 they were 44.8%, 44.4%, 42.3% and 23.5% respectively. So, at the top in on 0-2 he's not that far off one of the better k pitchers in the league but he's no where near as effective as Arrieta on 1-2 and 2-2. I'm not a pitching coach but one of the suggestions I've read is he struggled with establishing a 3rd pitch after AA.
Supposedly his change up was nasty doing this but they altered his mechanics some what to help improve his slider which worked but the change in mechanics really hurt that change up which hasn't been as effective since.
Long story short, I don't think it's really an issue that he doesn't strike guys out so much as it's an issue that he hasn't yet developed command and a feel for all his pitches. He might never develop them but his slider and sinker are pretty good pitches right now. If he gets that change up back he's pretty interesting.