[Lucid:]

it's because it's technically two separate events.
The masquerade had one currency because it was a smaller event and because currency could only be obtained by posting.
The rebellion has two, because there's a "common" currency (crystals) and a "rare" currency (jewels). This is so the items that cost jewels end up being rarer than the items that cost crystals when the event is over. this was done intentionally. the same currency granting system as this one was done during the Taskal War event about 6 months ago. the Nox Nocturnalis event last fall also had a common and a rare currency, though they were both obtained by posting. the grant ratios were just altered to make one easier to get than the other.
having two currencies with which to purchase event items helps the market so it doesn't get flooded with certain items. that's why we do it.