Why were Atmel MCUs used for Arduino? Atmel microcontrollers are not cheap compared to STM and Microchip. Why did Arduino select Atmel MCUs such as atmega328 and atmega2560 on their board out of so many alternatives?
Simply because the Wiring system that Arduino stole was written for the Atmel chips.

Arduino didn't "invent" the Arduino system - they just took an existing system called Wiring and adapted it slightly and branded it Arduino.