One was just given: Users don't really care to create an account to begin with, so they provide throwaway email accounts and low security passwords. If the apps required longer, safer passwords, then they risk losing signups.
If I get a message complaining about my password being to weak, from a service I might not care that much about, then there's an increased risk that I opt to not create an account.
Apple solution is actually pretty good, it allows me to quickly create an account to try out an app or service. If I don't like it, meeh, they only have the Apple login info and nothing else.