Why do softwoods grow faster than hardwoods? ...
Answers: This question (and most of the responses) are solely a generalization.
Softwoods, by definition, hold needles or scales and are gymnosperms. Hardwoods have broad leaves and are angiosperms. While most "hardwoods" are more dense (higher specific gravity of dried wood) than "softwoods," this is not universally so. Balsa, for example, is a hardwood and Southern Yellow Pine, used for flooring, is a softwood.
The growth rate also have nothing to do near strength or density. Slow growth ring-porous woods (e.g, oak) will have tightly spaced spring wood that make it weaker than slow growth. Slow growth pine has more closely spaced latewood and is stronger than hasty growth pine.
hardwoods have more mass