Voice assistants are a win-win situation. Just like most successful sites, you get the software free – in exchange for all your data. They're great for you because, they allow you to buy a phone minus the screen and the open-source software and the possiblity to customize it (which is great because you don't really need to make any descisions, which you don't want to make) or stop it from sending all your data to google (which is helpful because they can use it to enhance both the quality of services they provide you and their own bank balance).
But different companies have differing ideas about what constitutes a good voice assistant. And here's where we come to a split. Amazon believe it should be a department store – but minus the choice of retailer. Google believe it should, once again, connect users to information (of course, selling off the data to advertisers as well as ensuring that you're not connected to anything Larry Page views as "harmful"). Both of these companies also like to urge app developers in, to do all the work for them.
Apple on the other hand, probably only added SiriKit because everyone else had created a smart speaker – so they did too, and created SiriKit to go with it. What you see here is that Apple have closed off SiriKit. While google dialogflow allows you to interface with many, different, interfaces (such as Amazon Alexa, Telegram, etc.) Apple has put a big cross on that, prefering to make Siri an alternative to buttons (you don't click it, you say it).
It's not hard to see who will do better here.
Disclaimer: Despite being true, all of these views are my own and don't necessarily represent those of Google LLC, Apple Inc or Amazon inc.