FPort is a bi-directional protocol, using SBus RC in one direction, and serial telemetry in the other.  

I2C is a serial protocol that can operate at different speeds 100kHz, 400kHz, and 3.4MHz. Not all chips support all speeds but 100kHz is commonly supported. Speed is important as the data is transmitted serially, so a faster clock allows a quicker update.
I2C is a BUS - just like CAN and SPI - hence each devices can communicate providing they have different adresses. Some devices have fixed address , some are hardware preset (jumper or solder bridges) and many are software configurable.

The great strength of the protocol is that it only requires two wires, yet can have many connected devices and all of these can transmit and receive data at high speed. This saves a ton of pcb wiring.

the I2C protocol has an acknowledgement feature that means a sending device knows that a receiver has accepted the data. So I2C is more robust in a noisy environment.

USART a type of a serial interface device that can be programmed to communicate asynchronously or synchronously