They have a much shallower learning curve and a large moat of people who have used them for decades, simple as that. These days, AVR chips are a stepping stone

They have a much shallower learning curve and a large moat of people who have used them for decades, simple as that. These days, AVR chips are a stepping stone to learning embedded development. They're fine for very cash-strapped beginners, but it would behoove new developers to leave them behind quickly. They are more expensive and less efficient than modern options, and you won't find many well-paying jobs asking for AVR/Arduino experience. STM32 chips are a much better option, IMO, especially for beginners. They come in sizes ranging from <$1 Cortex-M0 chips up to dual-core Cortex-A/Cortex-M processors, and they use the same sorts of peripherals across all of those product lines. So you can start small, and incrementally learn about more complex parts and topics. And if you really need a breadboard-friendly chip, an MSP430 is probably a better option. It's a 16-bit MCU focused on power efficiency, and it has GCC/Arduino ('Energia') support. Some of them also have non-volatile FRAM memory.