Mirrorless is electrically and mechanially a MUCH MUCH MUCH simpler design (SLR is essentially two cameras in one, with the pentaprism allowing light to go to a second sensor used for autofocus, and also to your eye) and it has a lot of compromises in complexity, reliability, and performance.

Mirrorless allows the camera to use the image sensor itself to do all the work. That means autofocus can be much better, using every pixel on the sensor for autofocus. It means you can run computer vision software in real time to do fancy things, like finding out if there are eyes in the image - most likely, you want the eyes to be in focus. You can focus in near darkness, when it's too dark for you to see. And other stuff.

The biggest downsides are 1) the electronic viewfinder, these are not as nice for most people, but with the big caveat that they do allow you to frame an image in near total darkness where an SLR wouldn't allow you to do that and 2) the big one is battery life, it is 5-10X worse with mirrorless than with SLR.