Reason for choosing SPI based flash devices over I2C ones
Bandwidth. I2C tops off at 3.4MHz. SPI doesn't break a sweat at 50MHz. Obviously clever routing is required if you want to reach that speed, but it is possible.

Also, multiple channels. SPI can work in "quad mode", where 4 bits are transferred each cycle instead of 1. This results in a data rate of dozens of megabytes per second.