Target market may be a factor here as well? Notice where the rollout started: India[0]. This goes along with Google's push to support the Next Billion Users[1]. I'm sure the team working on Allo and Duo want to support people that move regularly between Desktop and Mobile, but at the same time they want to reach out to people that are just starting to connect to the internet. Allo's initial target market may not be the HN crowd.

[0] https://news.ycombinator.com/item?id=12545596

[1] https://www.techinasia.com/googles-reach-billion-users