If I remember correctly, it was only the script that was different. Then it shouldn't matter, should it since they communicated orally?I don't know the canon explanation, but the only theory I can come up with is the same reason human scripts have changed over the years gradually. Since the hexagons turned to circles, I might actually make a conjecture that the Voya Nui script is older, and over time the hexagons turned to circles. It seems unlikely that a script would become more detailed and complex.