I really don't understand Google. Wasn't Allo a tent-pole item at Google I/O a year or so ago, and now it's no longer being invested in?
Google has developed a habit of trying multiple approaches to any given thing, and keeping what's most successful. For definitions of "successful" that aren't always what the rest of us might expect.
My first message went to my wife, who's on an iPhone. It was really difficult to suss out exactly how Allo wanted to handle this conversation... It seemed to both want to fallback to a default SMS while also trying to do some of the Google Assistant things, failing at both.
There are certainly some countries where the vast majority of users are Android, but it seemed pretty unfathomable to me that they'd push a product that didn't function appropriately cross-platform.