3mm is where it all started, as this was what was readily available ~10 years ago - small tolerance 1.75mm just wasn't around in abundance...

3mm requires a lot more torque to move than 1.75mm - As the early reprap-esque printers all use big geared wades or similar extruders, the torque to move 3mm wasn't an issue either. Or they just used big heavy motors, which is why they all stayed relatively single-extruder based; as 2 x 3mm capable extruders in direct-drive is going to weigh a *lot*...

3mm requires much slower print speeds or much higher power heaters - the volume of 3mm filament in a melt chamber is considerably greater than that of a 1.75mm chamber - so more energy is needed to melt the filament in a timely fashion; or you have to lower your extrusion rate to allow more of the plastic to become consistently viscous.

3mm also had greater 'tolerance' room - if you allow 2% tolerance, then it was ok to produce 2.95mm filament or 3.05mm... meaning you didn't need to have the highest quality manufactured filament.

1.75mm offers the counter point to most of what I outlined above:

You need less torque, so can either use smaller motors for direct-drive filament extrusion (such as the standard SD's extruder which is a around about a .7a stepper...)
You can have smaller/lighter motors, meaning you can run multiple extruders on an axis.
You need less torque, so a given motor can also extrude faster.
You have a smaller melt zone, so filament can be extruded faster and there is less 'wastage' during color changes/transitions.
... and manufacturing tolerances have caught up to a point where 2% on 1.75mm is quite doable, making it plausible to reliably source 1.75mm that was relatively constantly 1.75mm (and not blowing out to 1.8mm and causing jams in a 1.78mm barrel...).

In todays terms, 1.75mm is the new 'standard', and 3mm is a throw back and legacy diameter - it doesn't offer anything but drawbacks in comparison to 1.75mm (and yes, you can compensate for many of those drawbacks, such as using bigger motors, higher power cartridges, higher geared ratios on a geared stepper... etc... but why bother if you can just go 1.75mm and avoid the issue )...