Why do apps not work properly?

"Based on data gathered from over 1 billion mobile users, Crittercism has found that even though iOS 7.1 is Apple’s most stable release for apps yet, the latest software still has a crash rate of 1.6%. On the other hand, Android versions KitKat, Ice Cream Sandwich and Jelly Bean all have crash rates of just 0.7%. Even Gingerbread appears to be more stable than any iOS version before 7.1. All in all, your apps are significantly less likely to crash on an Android device than on an iPhone, iPad or iPod touch."

I know nothing about programming but I am curious 1) what are the primary reasons apps crash and 2) why are apps released when it is apparent they do not work properly? It seems like Android apps would have more problems due to being installed on a variety of hardware but apparently this isn't the case. Also I have read that Apple has stricter standards for their apps but I've come across several iPad apps that don't work properly. If someone could explain in terms a non-tech person can understand I would appreciate it;