If I understood correctly, it was postponed because IOTA is not ready yet. But how do you want to make sure that IOTA implemented the features by the end of May? IOTA has always delivered but almost never at the specified time, so I remain skeptical whether the Mainnet comes at this time.
Since IOTA has not yet implemented this feature, the Oyster development team decided to move to Oysters private Tangle. By using a private Tangle, Oyster can implement sharding into our infrastructure, allowing the protocol to be production-ready without relying on the implementation of sharding into IOTAs main Tangle. The ultimate goal for the Oyster protocol will be to merge back into the main Tangle when sharding is ready, but for the time being, using our private Tangle allows Oyster to be a commercially available product.