There are two different measures taken to conclude the overall height of pants- inseam and crotch. The inseam tells us the length of leg and the crotch gives us the length of the crotch seam and therefore height of the waistband (rise). Modern pants are often much lower waisted than pants 50 or 100 years ago. Some of those styles still exist however, such as the standard dress pant for a tail coat. While the waist will change style to style, brand to brand, that's your preference. You'll always be able to easily find pants the right length even if they fit no where else, but if the outseam was given instead you'd have to know how every style of pant worked or try multiple sizes every time.

As for the history, it comes down to those measurements needed by a tailor to pattern or alter the clothing to fit you. They'll take inseam, outseam, waist, hip, knee, and ankle. If we're dealing with theatrical costumes, which may deal with being much more fitted, the crotch seam is included and possibly crotch depth (waist to chair when seated). In order for the outseam measure to work you have to know exactly where the waist will hit on the customer. It's the first line drawn in drafting and everything else is based on it. If I was to take this pattern and try to standardize it, we would define how the leg fits and how the torso fits separately (ex. boot-cut, low-rise) based on our companies "average" persons measurements. Both parts can be adjusted to other styles now and the outseam will vary based on both parts, being less consistent. Thus the issue with mass-production and why inseam, which is just leg length, was chosen over outseam, which combines both leg and torso.

Seems to me that it would be the opposite. The outer seam varies between styles, how high they are meant to be worn, how baggy they are, etc. Inseam is crotch to hem, which should be fairly consistent, unless you want cuffs in one pair and not in another.