Why do mileposts on US Interstates increase South to North instead of North to South?

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I assume the numbering of mileposts is to match the numbering of the U.S. Interstates themselves -- west to east interstates, which have even-numbers, are numbered from south to north (i.e. I-10 runs through southern states, and I-90 runs through northern states). Likewise, south to north interstates, which have odd-numbers, are numbered from west to east -- I-5 runs along the west coast, and I-95 runs along the east coast.

Note that this is the opposite of the US Highway system -- north to south routes grow larger from east to west. US Highway 1 is on the east coast, and the old US Highway 99 (which is now no longer a US Highway) ran along the west coast. Likewise, US Highway 14 runs through several northern states, and US Highway 82 runs across several southern states.

I believe the reason the Interstates were numbered opposite of the US Highways was to avoid confusion, i.e. when talking about route 5 along the west coast, you know the person is talking about an Interstate, not a US Highway.

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Comparing milepost numbering to the route numbering of the US Interstate Highway System is wholly unsatisfactory, as the practice of numbering from south to north predates the Interstates. At first I thought it was following the precedent of US 1, which famously has mile marker 0 in Key West. Then I thought maybe the tradition went back to the Boston Post Road, but then I realized that many of these early American roads (the National Road, Lincoln Highway, etc. in addition to the Boston Post Road) had literal milestones that marked the distance remaining to cities of general interest. So, for example, on the National Road traveling westward from Baltimore, one would see the miles counting down to Wheeling, WV; then Columbus, then Dayton, and so on. But eastward, they would count down to these same cities in reverse order, finally ending in Baltimore. So clearly this wasn't it.

Finally, I came across another website that made the point that mathematical graphs count up as one moves to the right on the X axis, and upward on the Y axis. Plotting this along most maps, one would see numbers increase as one moves to the north and east. So it seems that's the answer: Because highway engineers decided to follow the convention set by mathematicians.

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