Most of the IEM explosion is centered around Shenzen where the production time is very short to turn things around. They can buy off the shelf drivers (almost none of the drivers are proprietary), 3D print the shell, and keep prototyping without breaking the bank. Designing and producing a full sized headphone prototype is significantly more resource intensive.

Also I think many of these small IEM companies in Shenzen have learned from others and it allows them to churn out IEM's at a much faster pace because they have a lot of the basics down.