I was at a meeting the other day and it was thrown out that they are easy to remember. I don't know if you'd call the slogans platitudes, but lots seem to like them, and I find them less than pleasing.
There are a couple good ones though -- progress not perfection, it is what it is.
The easy to remember is important because when someone comes into AA often their cognitive functioning is at a low. Therefore, these catchy phrases serve as reminders or helpful hints that are easy to process, as someone battles the urge to pick up a drink in early sobriety.