Plo Koon was relatively popular in the expanded universe (his role in the 2D Clone Wars is minimal though, he appears only at the council scene and has no lines). Plo appeared in games like Jedi Power Battles and was a part of the prequel era ancillary material since before TPM came out. He even used a yellow lightsaber before that was retconned by Lucas in AOTC.

In Star Wars even the most unknown of characters can inspire a cult following, so I kind of understand why Dave Filoni would be intrigued by Plo Koon and wanted to turn him into a fan favorite.