One reason is that with a mirrorless camera you have to have the sensor powered all the time. That means background heat in the sensor which traditionally been

One reason is that with a mirrorless camera you have to have the sensor powered all the time. That means background heat in the sensor which traditionally been a source of noise. With a DSLR you only have to have the sensor on to capture your final image. That seems to be less of a problem with modern sensors so it may become a non-issue as time goes by.