High expectations for minimum wage. You are responsible when things go wrong. You are expected to solve everyone's problems. You are expected to be at the beck and call of your supervisor any time something needs to be filled. You can literally do 100 other different jobs and make the same wage with less stress, not have to deal with junkies, and not have to hold and maintain a licence to get paid peanuts.
It's dirty and gross work dealing with less than savory people and places.
It's not fulfilling and makes you jaded and bitter. At least cops get compensated handsomely.
Security has a poor public image. Why would you want to get harassed by the public for minimum wage? Paul Blart mall cop anyone?
Most other guards are incompetent. You get the odd forward thinking, pro-active, and ambitious guard, but most guards struggle to complete simple tasks. When you do get a solid, hard working guard, they quickly lose all of what makes them so great because what they possess is not appreciated or compensated appropriately.
You have to work nights. Again, why mess up your social life and your health when you can get paid minimum wage to do a day job?