Because the author didn't know how to connect the display to the RPi. Or, it was much easier to use a library that the author found online to display with Arduino vs writing his own on RPi. From my experience, there seems to be much better set of libraries for small graphic displays, seven-segment displays and alphanumeric displays for Arduino vs raspberry pi. Many people think the RPi is limited to an HDMI display or Official Raspberry Pi display. I could see a mini-OLED display being much easier and battery capable than using an HDMI display. If the author could have understood that RPi is very capable with I2C, SPI communication but may have been overwhelmed with also creating text characters to display all the digits on a small OLED display (already available from Adafruit snd other vendors for Arduino so no work at all via author's methods).