Sure. Let's start with the concept that HomeKit can only support accessories that it's been programmed to support.
Below are the accessory types (in iOS 10) that HomeKit supports:
You'll notice that SPRINKLERS is not on that list.
HomeKit in iOS 10 (and every version before) hasn't supported sprinklers. Nobody (Rachio, Rainbird, Toro, etc) can make a HomeKit sprinkler system before Apple adds "sprinkler" support to HomeKit.
iOS 11 (which launched literally 7 days ago) fixes that. Take a look below at the accessories it supports, and notice the far right of each row: SPRINKLERS and FAUCETS are now supported.
Hopefully this explains the reasoning behind the thought that Apple has been holding back sprinkler systems like Rachio from being compatible with HomeKit.