On a traditional DSLR, Live View requires the mirror to flip up. That blocks light from reaching the camera’s dedicated phase-detect AF sensors, which normally provide fast autofocus. In Live View, the camera often has to focus using the main image sensor instead, typically with contrast-detection AF.

Contrast AF is slower because the camera must move focus, check whether image contrast improves, and often hunt back and forth to find the sharpest point. Dedicated AF sensors are designed to tell the camera both how much and in which direction to adjust focus, so they are much faster.

Compact cameras are designed to operate this way all the time, since they are effectively always in Live View. Their systems are optimized for sensor-based focusing, and their smaller sensors use shorter focal lengths with deeper depth of field, so less focus movement and precision are needed to get an acceptably sharp result.

So the main reasons are: DSLRs lose access to their normal AF system in Live View, contrast AF is inherently slower, and compact cameras are built around Live View focusing from the start.