First, is the unattractive salaries and working conditions, especially in poor urban schools that result in many teachers leaving the profession for better paying positions and without the stress of the classroom. They are usually replaced by "newbie" teachers who make minimum salary, who in turn, leave for greener pastures themselves in a few years. This further depresses salaries as school districts suffer high teacher turnover and leave before receiving significant salary raises.
Second, State and Local teacher evaluation, complete with "junk science" , has made lasting in the profession a punitive one and has resulted in few college students willing to enter the teaching profession which has resulted in widespread shortages.
Third, the school and teacher accountability policies by politicians and education reformers have resulted in high teacher turnover nationwide. Many teachers decide that their talents are better compensated and appreciated in other professions.
Finally, the climate of disrespect fostered by education reform organizations and their political and media allies has made the teaching profession an unappealing long-term job and has encouraged potential teacher candidates to seek other professions.
A side note, having a union that can collectively bargain helps somewhat as unionized teachers make only 11% less than other professions with similar education and experience. However, that is sill far less than what they should be paid.