They hit the right combo of cheap ($€), accessible (you can buy them everywhere, shipped everywhere, even in quantities of 1), easy to use hardware (they're sol

They hit the right combo of cheap ($€), accessible (you can buy them everywhere, shipped everywhere, even in quantities of 1), easy to use hardware (they're sold on many different 'breakout boards' with usb connectors for power and programming and marked pins), easy to use software (documentation, examples, arduino ide, nodemcu) and without any weird limits licencing wise. Arduino was similar at first, but stayed on low-performance avr chips for way too long and non-chinese ones were expensive (and well, no wifi), rpi pico has some nice features, but much harder to get at first, and everything else is a "raw chip" bought on sites like digikey, with expensive shipping, 600+ pages of documentation, 300+ of them needed to send a first ping, you need to solder them onto a board and programming usually requires some expensive rig.