In my opinion geography plays a huge part in many people's reason to stay. Some of this depends on specialty but for the most part the coasts are looking to decrease their cost and trying to find degrees in other locations. However there are many cases where people simple do not want to move to the coasts. People with STEM degrees are not stupid generally weigh the benefits of moving to California, Texas, or New York / DC.
I have had several colleagues choose to move to the east or west coast. Some who have even moved back after a stent because they simile didn't like something about those places. There is also the cost of living that can change drastically. Trying finding 10 acres or 5 acres of land near San Francisco, Seattle, or New York. You won't not with out having a sizable commute. You can do that in the Midwest (well not Chicago) and to some degree in Texas, but even Texas is starting to show strain due to traffic. As much as I complain about traffic in Kansas City it is nothing compared to Texas, San Francisco, New York, or Seattle.
I have seen a number of tech workers wishing to be able to work remotely. This has strong pull to many but it is a new paradigm that many companies are not equipped culturally or technically to handle. However that is changing. I have seen startups and some large name places coming to understand that they can do work remotely and this will remove the geographic issue to some extent. In the past I have told several companies that I would love to work with them but I am in no position or have no desire to move to their city / state.
This of course has it's own trade offs. Companies in the Midwest are just now realizing the career options that companies like Google, Linkedin, Facebook, or Amazon offer to STEM employees. Otherwise they have been traditional limited in advancement options and frankly most people I know that are successful in STEM are generally drive to want to advance.
In short STEM will see better utilization when geography is removed from the equation as geographic markets play a huge role in the decision to move to the various STEM "meccas" in the US.