Several reasons: * Until fairly recently, cameras couldn't focus well using just the sensor. The simple sensor-based focusing is based on contrast, and the pro

Several reasons: * Until fairly recently, cameras couldn't focus well using just the sensor. The simple sensor-based focusing is based on contrast, and the problem with the method is that the camera can't tell whether it's as focused as it can get, or in which direction should focus be changed. So the camera constantly "hunts". You can see that happening a lot on youtube with people trying to show stuff from close (eg, Bosnian Bill, AvE very often have this problem). * Phase focusing fixes this issue, but initially couldn't be incorporated into the sensor. The mirror in DSLRs bounces light into the better focus detectors, which are located on the bottom of the camera. Here's an illustration of the system: https://photographylife.com/how-phase-detection-autofocus-works https://photographylife.com/how-phase-detection-autofocus-wo... * Sensors get hot when they're constantly lit. Cameras refuse to keep taking pictures when it overheats. A DSLR's sensor can cool off while the mirror is blocking it. * Sometimes the viewfinder is the better way, eg, if the LCD can't overpower the sun. Making a good digital viewfinder is harder than just optics. * An optical viewfinder has no lag and almost zero battery usage But yes, mirrorless is definitely the future. The DSLR was a technical compromise, and in fact mirrorless has advantages of its own: * As this article says, DSLRs require a large separation between the sensor and lens to make room for the mirror, and that makes some kinds of lenses hard to design. Mirrorless cameras can use whatever distance works best. * Also, the mirror is a pretty delicate, very quickly moving piece that creates vibration, and this needs taking into account. It's common to incorporate a delay to allow for vibration to settle, or to use a special mode where the mirror is permanently kept up for the cases in which vibration needs to be minimized. Mirrorless cameras don't have a moving mirror, so this is not an issue.