Not sure tbh. There are various theories from what I know. Something to do with node RPC replay (whatever that means) or something to do with too many transactions creating a bottleneck that crashes servers. These are the only 2 theories I found that made sense.... Somewhat....

Also there's one more theory that made sense. It is done intentionally to prevent large scale dumping of steem/sbd as well. But I doubt that blocktrades would do that so I'm sure there's another reason for there being no Steem/SBD exchange on blocktrades