Binance and bittrex is have secured platform that's why people choose this two and as we can binance list coin that have a good value and potential to grow in the future.
Most of the get rich quick noobs don't care about the security of the platforms, and considering that the majority of the people using these exchanges are get rich quick noobs, it makes even more sense.
I'm wondering though, how exactly do you as user know how secure a certain platform is? Aside from the basic log-in and withdrawal security measures that aren't anything special, there isn't much to see to be honest.
The only thing you can do is hope they take their overall security standards as high as possible, and that's it. It's impossible to say that one exchange is more secure than the other just because it is more popular.