My feeling is one reason is that few teachers bother going into administration because they are in the field for the KIDS, not for the power, perks, and pay, and any job that would remove them from interacting with kids is undesirable. Another reason has to do with the rampant nepotism and patronage that goes into the hiring process to begin with, so people who are in administration are not there because they are particularly qualified but because they are somebody's friend or relative.
About embattled NYC principal Marcella Sills, who was totally unqualified to be a principal or a teacher in the first place:
Teachers begged city officials to investigate “School of No” Principal Marcella Sills soon after she started in 2005 — citing her constant tardiness, harassment of staff and extravagant spending on parties while the school lacked books, pencils and paper.
“Get rid of her before it’s too late,” a 2007 letter urged District 27 Superintendent Michelle Lloyd-Bey, who oversees Queens principals.
Letters describe Sills as a tyrant and “rude lunatic administrator” who spurred an exodus of excellent teachers and failed to provide basic student supplies and services while handsomely furnishing her own office and squandering funds on catering and decorations.
Of course she won't be fired despite the outrage, for to do so would be an admission that HER superiors weren't doing THEIR jobs.