Gil, Materia, and RMT bots
ARR and HW dungeons drop a small amount of gil (~190 for Level 50 dungeons, ~320 for level 57, divided amongst the 4 party members) when you kill a trash enemy, while SB/ShB/EW dungeons have a low chance to drop a combat Materia instead.
Most people don't really think about the gil, but the majority of RMT bots will level up a bard with a name like "Kjydlr Yeqbhn" or a similar keysmash name and spam the highest level dungeon obtainable at the earliest point possible, before buying the game to be able to dump their gil on the account that's facilitating the operation.
Which happens to be The Wanderer's Palace, since it's the only lv50 dungeon that you can do before completing the Seventh Umbral Era questline.
Hang around any given early MSQ area and you'll see a lalafell phasing out of existence. Or in the East Shroud and see that enemies are being attacked from underground. It's an RMT bot.
ARR/HW dungeons don't drop gil when queued for through Duty Support, so it seems like SE knows that people might abuse dungeons for gil. So why not change all ARR/HW dungeons and make them drop Materia instead of gil?
It's a pretty simple argument. It cuts off the main source of RMT gil and stops a LOT of chargebacks from occuring, doesn't hurt anyone who's playing the game under normal conditions, and also puts a lot of Materia I/II/III/IV into the economy, when Is are the hardest to come by and II/IIIs are frustrating to obtain outside of a few Free Company ventures.