How did we decide that the North Pole is at the "top" of the Earth and the South Pole at the "bottom"? Couldn't we have oriented ourselves the opposite way?

There's actually a FAQ section on this question, though it hasn't been asked for a while now.

The answers dotted around the two threads listed there go to some trouble to point out that maps have not always been oriented this way, and in principle there is absolutely no reason why any one of the four cardinal directions should have priority over the others to be represented as "up".

This post by /u/khosikulu and this one by myself were the only ones to suggest a reason why one cardinal direction ended up pointing that way on maps: Claudius Ptolemy's Geographia, which was written in the 2nd century CE, and became very influential on the development of cartography in the 15th century after being published in a (rather bad) Latin translation in 1475. Here's an illustration from a contemporary copy of the Latin translation; this version is probably the most famous one; and here's a zoomable reconstruction of Ptolemy's data based on the Greek text as it was known before the new 2006 edition.

Ptolemy's reasons, as I lay out in my old post, were simply that

he had more data on the north-west of the Europe-Asia-Africa landmass; and
Greek was read ~~right-to-left~~ left-to-right and top-to-bottom in his time.

Therefore, the more detailed bits of his atlas (the north-west) went at the top left. Hence north was "up".

Other ancient astronomers, like Kleomedes, used a different orientation; so did many (maybe most) western European maps prior to 1475. But Ptolemy was especially influential, and Mercator's map -- even more influential, if we take the long view of things -- seems to have been designed as an attempt to definitively supercede Ptolemy.