I'm not a music theory person, but I've played guitar obsessively over the past 10 years, so I have a good grasp of practical applications of the instrument.
It seems like the overwhelming majority of the songs I've learned on guitar (ones written from the 1950s-present) are tuned to Eadgbe. Most of the time this is just referred to as "standard tuning". If I had to put a percentage on the percentage of songs from 1950-present where the guitar and bass have standard tuning, I'd put the number at around 80% - extremel, extremely prevent.
What confuses me about this is that other tunings don't make the instrument sound drastically different. You still know it's a guitar, and most people would have no idea the tuning is different unless they play the instrument. You'd think there'd be a lot more variety.
Stringed instruments have been around a long time, but when did standard tuning become standard?
This isn't really a history question, because the reason for "standard tuning" is less a question of historical impetus and more one of functionality and physics.
The size of the guitar was more or less standardized in the late nineteenth century, and as a result due to its physical dimensions, there is only so much tension that can applied to the strings without it snapping the strings or being too loose or flubby to sound good. It so happens that this range, when using gauged strings between .008 to .013s for the highest string, and .038 to .062s for the lowest has an ideal tuning around the frequency of 82.4hz for the lowest string and 329.63hz for the highest. Which corresponds to the note E, though one can adjust this within a step and a half or so.
As for why the rest of the notes are the we they are is just due to the nature of guitar as a fretted instrument. Being tuned a perfect fourth apart (with the exception of the major third between the third and second string) allows for the most maneuverability on the fretboard. You can reach the most notes being a perfect fourth apart.