There were two problems that had to be overcome.
- The weight. Plows weighed quite a bit and only oxen were strong enough to pull the plow. The early plows were made of wood to make them lighter until the second problem could be solved.
- The plow clogged. Usually plowing required 3 people: 1 to guide the team, 1 to guide the plow, and 1 to clean the blades. It could be done by one person, but it just made things slower.
So the problem seemed to be technology. They waited for the technology to address the problems they had in the fields. I guess the best answer is that metal plows were in short supply, but the real reason lay deeper.