Why We are Using CSI2 Instead of Another Interface Like i2c or USB?

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Depends on the application....

..."The biggest issue with USB is bandwidth and processing needs."...

http://petermoran.org/csi-cameras-on-tx2/

Thank you for your response, but i still confuse.. for example: like if my device 1 want to transmit 24 bit pixel to device 2.... so if i use 24 paralell lines for this transmisson instead of csi2, can i get better result

Probably not, and one does not preclude the use of the other, but it depends on the application, the set-up, the layout, the components being used, the context and a host of other variables....what are you building and what is your goal?

https://www.raspberrypi.org/forums/viewtopic.php?t=141682

http://paulbourke.net/dataformats/bitmaps/

CO$T and COMPATIBILITY (unless new product) are two major driving forces.

Well, it all depends on what you are connecting together using this interface. You forgot to mention that.

Sometimes a special purpose interface is chosen to prevent extraneous devices from causing time delaying collisions somewhere in the data stream. Sometimes legacy dictates doing it that way again despite the improvement of other technologies. Sometimes it's a flat out dumb choice that is wasting money.

Presumably it's because you are working with data from a video camera. CSI-2 stands for Camera Serial Interface 2, which is described as "A widely adopted, simple, high-speed protocol primarily intended for point-to-point image and video transmission between cameras and host devices"

Do you have links about mipicsi2 protocol,please share me, thank

You may find it a novel suggestion, but how about searching for the information yourself? Just type "mipi csi2 protocol" into your favourite search engine, and you may find that CSI-2 is based on I2C anyway.