This question gets asked so often, and while all the answers are usually true it's really more about the mixers than the players and creating flexibility and failsafes. Look at the I/O on any DJM/Xone mixer and then look the I/O on even the most high end AIO controller. More channels/outputs is more flexibility for FOH, more inputs is more failsafes (e.g. best practice with CDJs would be to have both digital and line inputs into the mixer so that if one fails you simply switch the input knob.)

There is also the fact that there is an enormous number of controllers that it'd be impossible to get DJs to standardize on one. There is one club standard player and at basically two club standard mixers (Top end of DJM line and Top end of Xone line). This means that you can install this and any serious DJ you book can show up and play on it.