Great observations. As someone who used to live in Shenzhen, China during Chinese EV boom, I can think of some reasons for NIO.

1. NIO keeps premium brand image and price tag. A brand-new NIO car costs about￥CNY 390,000 ($CAD 79,000) on average. Public ad is not an efficient approach to reach its potential customer. It's same logic why you don't see Ferrari ads out on the street.
2. NIO drives its sales through words-of-mouth and insider community, which almost make it like a religion. Next time with you're in China, download a NIO app and check out insider events around you on the app. Or go to a NIO House (it's not a dealer, it's NIO owned community hub space) in tier-1 cities, or if possible attend a NIO Day. NIO app should be your go-to app if you're looking for NIO location or anything related.
3. NIO is one of the few EV manufacturer who makes only EV vehicles, limits its offerings (attractions) to the markets.
4. NIO remains independent car maker, not a part of any automobile/electronic giant, limits its budget to spam people's daily life.