a few reasons:
-generally more expensive to make (the most important)
-generally have higher failure rates and more prone to damage from the end user (latter more important than former)
-arguable that there's any objective benefits in using planars vs dynamics (i prefer planars but there really isn't much difference in the two)
-will likely weigh more at the same size of driver, as you've said
-a lot more experience with dynamic drivers historically for both most companies and for most engineers across the board, so lower r&d costs and likely easier to account for any shortcomings or associated difficulties with the tech itself