Few things...
1, they can be flush mounted because of the front port. This is a useful thing in a large studio which is the use case for these.
2, they go louder. 115dB/1m continuous per speaker is truly monstrously loud (and this makes sense, as they are midfield designs rather than nearfields).
3, ATCs have a very long reputation for being damn near indestructible - drivers and amplifiers both. The Ones haven't been around that long and their long-term reliability track record is not known (this is not to suggest they're unreliable, but rather that they're a newish product).