I’m not particularly savvy with a lot of the technicals but I also know that lens design has improved with many mirrorless brands because it can be mounted closer to the sensor without the mirror getting in the way. This allows for better light collection and other advantages over a traditional mirror box.
This superior optical design is one of the keys.
Cameras in the late Nineteenth century were built where the rear element was very close to the focal plane. Think of the bellows-style cameras like a 4x5 view camera. By putting the rear element at the focal plane, this allows for a nearly-symmetrical lens design where the cross-section of the front half of the lens looks like a mirror of the rear half. This is particularly important for wide-angle lenses.
The reflex mirror in the SLR (and other reflex mirror cameras) prevents the rear lens element from being adjacent to the focal plane. That requires a retrofocus lens design that introduces complexity and worse increases lens size; this is basic optical physics.
Most of today's great mirrorless lens designs were invented in the late 19th century. With digital mirrorless cameras, many manufacturers can return to these classic lens designs now that the pesky reflex mirror is out of the way.
Note that this is not specific to digital photography. Mirrorless 35mm film cameras also enjoyed vastly superior optical designs and smaller lenses. Look at the 35mm rangefinders like Leica and Contax cameras.
You can see this comparing lens designs for 35mm SLRs (retrofocus) and 35mm mirrorless rangefinders. At the same focal length, the SLR lenses are massive compared to the equivalent rangefinder lenses. The former also have more performance problems (distortion, light falloff, etc.). A 28mm Distagon wide-angle lens is HUGE. The 28mm Biogon wide-angle lens is tiny. The Biogon lens design is vastly superior to the Distagon but it can't be used in an SLR.
Even the same lens design in 35mm SLR vs. 35mm rangefinders is vast. Look at the size/weight difference between 50mm/f1.4 Planar for Contax SLRs vs. the 45mm/f2 Planar for Contax G-series rangefinders.
Whether you are a pro or amateur, you have to think whether or not you want to carry around a bulky body and humongous lens versus a slimmer body and smaller lens to capture a photo at the same equivalent focal length. For a pro, pitching that four pound 300mm/f2.8 (or 600mm/f4 honker) sure starts to look appealing if you're shooting a three hour football game.
Now that digital cameras have moved to digital (non-optical) viewfinders, a wider range of lens focal lengths are available to digital mirrorless cameras. This wasn't realistic for 35mm rangefinders with built-in optical viewfinders.
