I think the answer is obvious. Hydrogen storage is a technology the world has yet to crack, at least economically. Here's a wiki link that shares the basics, http://en.wikipedia.org/wiki/Hydrogen_storage
Ammonia on the other hand can be stored in anything your heart desires, can be bought at about any store you walk into, and would not cost billions of dollars to build out infrastructure in support of the Hydrogen economy. Did I mention even a kid can buy a gallon of ammonia? That left on shelf its volume stays the same? That one needs not concern themselves that its explosive?
Governments and industry continue to pour millions of dollars trying to lower cost of storage, with this potential tech that quest could become null and void, IMO.