[hazeaj]

I think it's because of the competition, instability and well, money. For instance, when an extremely important exploit is found on Android, it has to be pushed by Google to all manufacturers which is almost impossible considering the amount of brands there are. There are also a lot of sponsorships by companies which force them to put bloatware in the phone. Also, every phone manufacturer wants their phone to stand out. Regular Android is too "vanilla" for them so they go make a (most of the time) crappy, ugly, unoptimized piece of crap.