Why we can wake Arduino Uno from sleep using only pin 2?
It's not just Pin 2 that can wake from sleep. Depending on which sleep mode you are in different interrupts (both internal and external) can wake the chip up.

The datasheet contains this helpful table:



The external interrupts (pins 2 and 3 on the Uno, and only "Level" trigger mode), pin change interrupts, I2C address matching, and the watchdog are the only interrupts that can wake the chip from any sleep mode. Other interrupts only work in certain sleep modes since they require parts of the chip to still be running in order for the interrupt to fire.

So that means that yes you can use any pin you like (using the "Pin Change" Interrupts) to wake the chip.