The cost.
I’m no expert when it comes to digital servos. I’ve flown for more than 20 years & have never used one. I believe they are faster, stronger and more precise. They convert the PWM signal from the Rx to a number and hold that position until the next signal comes through. An analog servo uses a pot to interpret the signal.
Professional pilots can make use of the benefit from a digital servo but for merely mortal pilots like myself, analog is good enough.