Preferred by whom? Where I live, my phone has to be turned on, find the carrier, try to connect to Tesla, which then has to wake up the car. This takes over a minute. Compare that to just walking up to the S with the fob in pocket or purse. Time: about one second. Same with backing the unoccupied car out of a tight parking space. A couple seconds hold on fob, car flashes, starts backing. Phone takes a minute at least. Bluetooth? The fob can reach the car from across the parking lot.
Not everyone has a smart phone. I got one when I got the S. But the fob takes up a lot less room in a pocket.