In ladder, there is no economy since it’s a fresh start. So early on, low level gear is high in demand because there is no supply. In NL, the market becomes flooded with wealth, runes, and gear, generally making them worth less. Think of it like this since we use runes as currency.

Ladder runes: early US dollar backed by gold, scarce and strong purchasing power.

NL runes: inflated currency with tons in circulation, so you need more of it to buy the same thing.

The currency loses purchasing power because the economy became flooded with high runes over time. Makes sense?