Lots of things
1) Lack of originality. They can't think for themselves. They end up copying from a company regarded as having good style, Apple. The only problem is that when Apple went Material Design (a/k/a) Moronic Design it was boring, bland and 2 dimensional. But that didn't stop the lemmings from jumping off the same cliff. MS followed with the hideous tile interface that they are still forcing on people in Windows 10. I don't know what the rest of the world thinks is attractive, but to me it sure isn't stupid monochromatic squares everywhere.
2) Lack of taste. This is exemplified by Material Design.
3) Lack of willpower. Not having the will to do something different, something that isn't derivative.