Hive is a British Gas product. It's in their interest to promote interoperability.
Nest, on the other hand, is firmly embedded in the Google Borg, and there's no impetus to make Nest equipment play nicely with Apple. Just imagine, for instance, if you could take a Nest Camera and make use of your iCloud data account for video. Google would hit the self destruct button first before that happened.
It's a shame, because the Nest thermostat is a lovely piece of design, whereas the Hive thermostat looks like my cheap light dimmer switch from the 1980s. I have a Nest thermostat myself, and the only bad thing about it is that it depends on Google for its existence. It's mildly annoying that we have these competing (and incompatible) standards. It would be great if the EU issued a directive saying that hubs must be designed to be interoperable.