The reason is... there is no reason. NASA is a bureaucracy that consists of smart individuals that collectively, do not end up making efficient decisions due to the fact that it is a jobs program. Zubrin pointed out with the MSR as an example that NASA keeps turning missions into visions. A delta-iv heavy could easily send a pathfinder size rover and MAV to Mars and get the sample directly to Earth. Instead, there are now 2 rovers, an ion powered satellite to grab the sample from Mars orbit, and a manned(!!!!!!!!) mission to retrieve the sample, by which it will be at least 2026, and Elon will be happy to lend NASA a couple dozen pounds.