Any future AWB that doesn't have a grandfather clause would be extraordinarily costly to enforce and results would be, at best, inconclusive and unverifiable.
If results aren't quantifiable and the government spends billions trying to seize every "assault weapon" in circulation, justifying an AWB to the population at large, considering the current fiscal issues, is political suicide for his party. Before committing to an AWB, Obama has to ask himself questions like "what happens if we do this very expensive thing and gun crime doesn't decrease? How do we explain that? How do we justify spending that money?".
Even the DoJ's very own report on the last AWB stated that the effect was so small it was probably immeasureable in large part because AW's are rarely used in crimes.
A total AWB would be akin to trying to put all the water back in the Hoover dam after it broke; there's simply too much, you'll never get all of it, you'll never know how much you've missed, it's going to take a long time, and it'll be expensive as hell.