Here's what I think:
1. Lack of status within the medical profession and general community. Stigma with the mental health profession, etc.
2. Psychotherapy. Not everyone has the capacity to listen to patients rant about their personal problems for extended periods of time. Most people don't go into medicine trying to figure out what it is that makes people cry at night.
3. On the job: not physically exhausting, but mentally exhausting. Ie. dealing with depressed, anxious, irritable, psychotic, paranoid people all the time. Some people just don't want to be in this type of environment on a daily basis. Just like some people don't want to be forensic pathologists looking at dead bodies all the time.
4. Ambiguity. Alot of Grey zone. Lack of definitive answers. Not as cutting edge in terms of research. Feelings of impotence as a clinician. Not as much immediate gratification from "curative" treatment plans. Alot of patients take months or years to get better. This is often times not observed by medical students rotating 5-6 weeks through Psychiatry.
5. About the salary (psychiatrists are in the lower tier compared with other specialties): There are not as many "business savvy" doctors who want to be entrepreneurial to get a "higher hourly rate". (ie, setting up the LLC, payroll, marketing, filing quarterly taxes, legal aspects, hiring secretarial or office management, accounting, business insurance, malpractice insurance personal health insurance, supplies, property/rentals). You don't need to start a business in medicine to realize you can potentially make more money running your own business rather than working for someone else who takes away half your paycheck in overhead and profit.
6. Right now market forces favor other specialties in terms of monetary compensation. If psychiatrists were getting paid more than anesthesiologists and surgeons at hospitals and people STILL weren't going into psychiatry I'd be asking questions.