Not sure why they chose 3.5 day wait but doing it instantly would create the ability to maintain both the peg and possibly stabilise Hive price alot higher.
You'll notice as Luna starts to dump it picks back up relatively quickly that is because people are minting my UST and then as UST peg breaks they convert back to Luna at a profit until the peg is maintained.
This constant back and forth creates a more stable token and stable coin.