It's to discourage people from camping high ladder. If not for decay, I could hit Top 10, not play another game, and I could come back months later with the only changes to my ranking coming from more people passing me on the ladder. To consistently be a top ladder player, you should understand how any given metagame evolves, and having to play games regularly enforces that idea. If I come back to a metagame in a state unrecognizable compared to that of when I left it, I'd have to spend time essentially learning a new tier, and a change in elo/ladder placement is designed to reflect that.
TL;DR it stops people from camping ladder and makes them know the metagame to keep their ranking, hope this helps :)