Why is there no hawkeye for the foot faul or ball out situations?

Question

If this choice has proven to be very popular in tennis, why it hasn't been introduced into volleyball?

Answer 1

I'm not sure what you mean by 'hawkeye', but at the highest level competitions, there are four line judges.  If you're referring to a computerized system of determining balls in or out, it's not really practical for volleyball.  The volleyball is larger than a tennis ball, and has less velocity than a tennis ball.   Each line judge has a dedicated line to call, and any one could also make block touch or antennae calls.  There are four eyes watching the server to ensure that he would not foot fault.