It's a bit strange, really. The Google Assistant on your smartphone is able to create reminders for you, but Google Home which has Google Assistant inside, is still unable too. This is just the latest example of how fragmented the Google Assistant is and it's not even a year old yet. Between Google Assistant on your smartphone, in Allo and in Google Home, they all offer vastly different experiences and features. Meanwhile, Alexa has had the ability to create reminders all along.
Obviously, not being able to say "Ok Google, remind me to..." to the Google Home is a pretty glaring weakness. Since it is much easier to tell Google Home to remind you to do something, compared to asking your phone to do it, especially if your hands are full. It's unclear why this is the case - as is the case about why the three Google Assistant platforms are vastly different - but hopefully Google will get this straightened out sooner rather than later. Google has the app support for reminders, with Calendar, Keep and Gmail, but for some reason reminders just isn't there. With the app support, Google is ahead of the Amazon Echo, if it just had reminders. Since the Echo doesn't have the app ecosystem behind it, all Alexa can do is remind you to do whatever it is, when it's time to do it.
These smart assistants are getting smarter and smarter, but it's still surprising how some of them lack basic functionality. Of course, this was also the case with smartphones when they first came out. In fact, the iPhone didn't have copy/paste for a few years, never mind a flash for its camera. So these are missing some basic features, but they will come eventually. The good thing here is that a software update can add features to these assistants, since the hardware will likely never need to be upgraded - at least not as often as a smartphone would, since it's just a speaker, and they are already pretty good. The Google Home is getting there, but obviously there is still some work to do. Google will likely have some big updates for Home coming later this year.