Some of the hurdles I see:
1. Faculty in university physics departments don't guide their students to think about teaching as a career (it's all about the Ph.D.-track students). Even the University of Maryland, an institution known for physics education research, only produces about 3 physics-certified teachers per year (based on recent info from the university president (or maybe the A/S dean) at the NASULGC meeting, which had a session or more focusing on their science and math teacher initiative).
2. The pay. I'm sorry, but with 1 credit short of a master's I was making ~24 grand.... I had friends who left with their BS in physics got jobs at companies for 35+. Some districts (like Denver) are making improvements with incentive pay, but certainly not all.
3. My god the hurdles to get certification! You have to go through a certification/licensure program at a university, take some national tests in pedagogy and subject area, and be licensed by a state process. And good luck getting complete stipends for the university program (mine, 10 years ago, gave tuition reimbursement, but I still had to pay living expenses out of pocket)... and those national tests aren't cheap either. And then good luck getting that license moved from state to state, especially if you haven't taught in that state for 3 or more years.
With my family conditions (middle-school age disabled stepson #1, elementary stepson #2, and additionally probably even another bambino on the way -- I've got to get myself to a doctor after the winter break!) I'd LIKE to be able to even consider teaching at the HS level again (being a part-time lecturer at the same university as my spouse, who's in administration now STINKS, even though my chair is amazing and I've started my own research program in physics education with a university grant). But note: With an expired Ohio license, I'd have to go through enormous hurdles to teach HS physics again (I taught in Ohio two years before receiving a better financial offer from the Air Force Research Labs, which included full stipend for an MS in Optics)... so even though I now have my M.Ed. in classroom teaching, in addition an MS in optics, and a Ph.D. in physics, doing outreach for our university to HS teachers giving them ideas for their classroom, and doing some pretty innovative physics education research (conference presentation coming up in May showing that, unlike most college classrooms, I've improved non-science students attitudes towards physics), I can't teach at the HS level. THIS I consider an outrage.
Principals should be able to freely hire (based on a CV -- with which licensure could be one credential but not necessarily required, and making their own offers of salary based on their districts funding). While I believe in unions for other causes, teacher's unions should not exist at most districts. With this approach, maybe university faculty would even change their views of pre-service teachers.