The actual reason was due to the (long closed) ARR Beta Test Forum being spammed to hell and back by a mass influx of new testers between phases 2 and 3 in 2013, so SE first introduced the posting restrictions there to try and bring some order to the chaos. Then, probably to head-off a repeat of that nonsense they also introduced similar posting restrictions here on the main FFXIV forum when ARR launched (which until that time had been for the old version of FFXIV - version 1.0).

Once again, a minority of foolish people spoil it for everyone else: "this is why we can't have nice things" and all that.

And speaking of 1.0, for those curious that is why the FFXIV forum has a 'five shades of brown' theme, it was to fit in with the theme of the old 1.0 Lodestone which had an old-fashioned look with backgrounds resembling aged parchment, and I guess SE never bothered changing the forum's theme to something newer when the game changed over to ARR.