HBO has historically been a channel that most people primairily get to watch movies commercial-free on cable/satelite and probably still is. HBO shows are critically acclaimed but you aren't seeing massive viewership via linear in the past two decades apart from a handful of shows, especially in recent years with streaming. Most of the viewership is happening on streaming nowadays for the people interested in the shows. The Last of Us only had around ~1M viewers on linear whereas had millions more viewers on streaming.

So to answer your question what's happening 6-8PM is that they are showing movies. Movies that are likely the primary reason why a lot of people are subscribing to and using the linear channel.