In my opinion the real reason was to stop arbitrage - do not have much of the track record with them, but the problem is that while "exchange" is decentralized, the gateway is not and at any moment somebody can just pull the plug. So "decentralization" is still controlled in arbitrary manner and may be abused at any time to generate an advantage to the one controlling it.

Ahh... the official reason provided at that time was "technical" just strangely enough this "technical" difficulties appeared at certain very convenient (or inconvenient for some) time 😉