Every company has it's "standards" and some managers are willing to make exceptions. However the following are fairly common reasons why people unable to maintain long-term employment.
1. Lack of a basic education. (Poor communication skills)
2. Lack of ambition/desire (Not looking for a job)
3. Unreliable (Often late to work/meetings, not completing projects, having a career is not a major priority, slacker)
4.Drug/alcohol/gambling/porn addictions (not focused on work)
5. Major felony criminal record (No demand for rapists, murderers or bank robbers)
6. Extreme tattoos and piercings or shocking hairstyle/color. (Offices look for "conformity" over "individuality" when it comes to appearance in the workplace. You have to "look" the part.)
7. Too many personal problems collide with getting work done (Can't get baby sitter, car broke down, relationship/marriage drama, bill collectors calling the job, garnishment of payroll for back child support or payroll taxes, tons of personal calls.)
8. Not a "team player". (Anti-social personality towards other employees, sexual harassment, disrespect authority of superiors, quick to anger, refuse to pitch in to help others)
9. Refusal to adapt to changes in company policies or fighting new training process.
10. Only interested in picking up a check. (Offers the company nothing in way of new ideas for increasing profits, reducing costs, or improving processes, not looking to move up in position; These are the first people to be let go in hard times.)
They're seen as dead weight.