Also, another technical explanation. As these microprocessors and boards started out in late 1990s, 5V logic was the norm. As advances in transistors and thus chip manufacturing, as well as the push for mobile devices and thus powered by battery lower power and thus lower voltage chips and logic began being built. This is why the name of 5V VS 3V because this is the basic regulated voltage the processor and memory chips run at those specific voltages. The lower the voltage, the lower the heat produced by a chip or conversely, the longer the chip could be powered by a smaller battery pack.
This is why there is that line in the sand.
Even today, you might hear about Arduino based electronics or projects. Those are mostly 5V based logic. Yes, there are exceptions and there are 3V (technically 3.3V) versions as well. These are used for the same reasons. 5V is just the older standard and still valid (many sensors and other items are based in 5V logic), however many newer sensors and processors are based on the 3V standard.
