I thought the lack of a magnetic field was considered to be such a "fundamental reason", i.e. without one away any significant atmosphere that's created will be stripped away by the solar wind.
The issue is timescale. Yes, the atmosphere is (very slowly) stripped away by the solar wind, so we could not simply terraform it and forget about it. On the other hand, the timescale is very long by human standards, so as long as we produced new atmosphere at a rate faster than it is being stripped away, I don't see it being a fundamental problem. I could be wrong, of course.
With a weak magnetic field, wouldn't something such as a solar flare, which the Earth has more sufficient protection against, possibly wipe out any part of the surface it touched on Mars?