Market forces tend to push companies to create artificial distinctions between themselves and competitors:
This drives consumers crazy, but it still happens. And it has happened with Zigbee.
Zwave opted for strong transparency: certified Z wave devices will be able to communicate at the “basic“ (that has a special meaning in Z wave) level, and everything else is optional. and “basic” is really basic: on/off for binary control devices and set level for multi level devices like dimmers and volume controls. And with the newest standard, new devices will have to support the S2 security level and lifeline association. But that’s it. You’ll be able to turn a switch on and off regardless of the manufacturer, but not much is guaranteed beyond that.
Zigbee is even looser, although they tried to tighten things up a little bit with the 3.0 standard. But then that ended up with a series of compromises. Just as one example, zigbee 3.0 hubs are not required to implement support for the Zigbee green power clusters, which is why the Hue Tap can’t be added to a smartthings hub. And 3.0 hubs are not required to support touchlink commissioning, although they can.
(I know that is probably all way too much jargon to be comprehensible, I just wanted to give a couple of specific examples for those who do dig down into the details.)
The main point just goes back to that picture of the charging cords: if a standard does not require interchangeability, market forces will generally drive companies to create features that their competitors don’t have. And that leads to integration issues for a multi brand setup.