Why Servo works only with PWM?
PPM is also available. so why only pwm?

Beacuse PWM is the simplest signal in which the pulse you give or the amount of duty cycle is directly proportional to the mean voltage in the signal.
there is a motor a potentiometer and some circutry in a servo. it just compares the voltages and the current position voltage on the potentiometer.

but on PPM the mean voltage does not change with different pulses.
so some other mechanism has to be adapted to make it work which seems more complicated.
(I'm not talking about CPPM)

and Cppm cannot be used because you need to control each servo with a single wire so you again need to split the continuous signal coming from the single wire.

@TunerX_98
PPM ESC can be possible? but why esc only with pwm?
Less amount of pins will be needed for ppm.
4 in  1 esc are available but not with ppm input.

Tiny or Small receiver have ppm output and doesn't have pwm output then 4 in 1 esc with ppm input will be useful
flight time will be more due to less weight.

Design yourself and submit please. We will send it to sevaral txrx manufacturar. Please take the challande of design. Some resourses and design idea is already available in this forum..

@Taksh.
From your questions it seems like you are asking questions without really understanding the underlying challenges.
1. The reason why ppm servo is not there because it is not needed to redesign everything. You still need one channel per servo if you design a PPM servo which means you just changed the way software works which means total rework and saved nothing. PWM was existing from long time and as Tuner_X(sri hari) pointed out CPPM is not at all an option. Indeed CPPM is split into several channels by receivers internally.
2. 4 in 1 ESC doesn't mean there is only one controller which takes all 4 inputs and control 4 motors. It is rather 4 ESCs which share common power supply, board and few other things. You have to understand the basics of ESCs so that you can understand all by yourself.

Any question you asked above is not tough to implement but not necessary. Moreover there won't be much analog communication. Communication would be digital in near future between ESC and FC.

@KALYANPRODHAN  @Tuner_X  @Balakrishna Reddy
First of all thanks for reply.

@Balakrishna Reddy 
I agree to point 1.


Upcoming posts are welcome.

Indeed it is already.
We already use OneShot, MultiShot, and DShot for FC ESC Communication on multirotors. (although oneshot and multishot are nothing but PWM implemented in a different way)
I agree to the point that we need not change the way software works as stated by Balakrishna sir. because incase of fixed wing aircrafts, anyways we need to give separate signal to each servo. for us to control all the control surfaces separately. so implementing PPM would do nothing well except changing the way things work.

Also as stated by Balakrishna sir, if we use protocols as CPPM or other 1 wire per channel protocols, then we need to use internal splitters for the signal to send an individual signal to each servo on the aircraft.