Some thing that always bugs me (especially these days when people talk about 4/3 digital sensors have a "2x" crop factor to FF), is that there's no *easy* calculation between format sizes when they're a different shape.

Take a 4x5 frame, actual image size is 94x120mm (numbers I got from a bit of googling) and a 35mm frame size of 24x36 (may be optomistic for film, but it's the size of digital FF at least).
The 4x5 has a ratio of 1.27:1 (near enough a 5:4 ratio), the 35mm has a 1.33:1 (6:4 or 3:2 ratio).

So what do you want your output image ratio to be?
Take the 4x5 and crop it to a 3:2 ratio (or anything 'longer'), then the image is 120x80mm. Then compare lens sizes to get the exact same framing and DOF your SuperAngulon 65mm f/8 is the same as putting a 19.5mm f/2.4 lens on your 1DX. 90mm f/8 is the same as 27mm f/2.4. That's a "divide by 3.3" rule of thumb.

But what if you want a 4:5 print size? (or 8X10, 16x20, or anything 'squarer'). Then you have to crop the 35mm frame to 30x24mm (and the 4x5 frame to 94x117, but near enough). To get exactly the same framing and DOF, your 90mm f/8 Super Angulon will give the same image as a 23mm f/2.0 on a 35mm. Your 65mm f/8 will give the same as 16.6mm f/2.0 on FF. That's a "divide by 4" rule of thumb.

Anyway, most of that is beside the point. Freaking wide is Freaking wide. (and that's wide, I've got the sigma 8-16mm on my digital-crop, widest it gets until they make a <43mm on 4x5). It's already been said, but "distortion" in the true sense is Barrel Distortion, think of what a fisheye lens does, that's barrel distortion. "Wide-angle" distortions are the "big-nose-small-ears", as well as "falling over buildings" phenomena (if your lens is not 100% horizontal). Both are made worse by barrel distortion, and both tend to happen together on the widest lenses because they're pushing the extremes of lens design to get that wide (especially on SLRs). Google the Samyang 14mm, or EFs 15-85 at 15mm, or EF 28-300 at 28mm for sample pictures of extreme barrel distortions.

As has been said, on SLRs they're all Retrofocus designs to clear the mirror box. Focal lengths of 12-35mm are less than flange distances of 40-50mm.
Luckily on LF (and rangefinders), you can get away with Biogon designs, symmetrical (or almost), not much room between the lens and film but doesn't have to be, and very very low distortion. I haven't got numbers, but I'm guessing that the Super Angulons of today do better than all but the best SLR designs.

Back to the OP, why is 90mm the 'standard'? Because lens designs back in the day of "angulons" could only get so wide in terms of degrees of coverage, and 90mm just happened to be the length that covered 4x5 nicely (wide open and/or with movements, whatever the market thought was 'good enough'). Flash forward and "super angulons" can cover a wider image circle, so 65mm can now cover what was only possible with 90mm before. It's even wider now, Super Angulon XL covers 4x5 and then some at 47mm focal length.