Recently, an interview with one of the developers went viral in the Diablo IV fan community. The assistant game director mentioned the approach to creating the game and additional content in a conversation with IGN.
According to Zaven Haroutunian, the development of the Lord of Hatred expansion began before the release of Diablo IV. This influenced the team's work; for example, a mode planned for the DLC eventually appeared in the game much earlier:
Throughout the development of Lord of Hatred, the number of planned activities changed – that's probably the best description of what's happening. Partly because we started working on [the expansion] very early; we were working on it in some form or another even before the base game's release, so it's been "cooking" for quite a while. And along the way, some things that were in prototype or development ended up being released earlier. We thought: "Well, yes, a mode with waves of enemies is cool, but it would be even cooler to release it earlier."
Developing expansions before the release of the base game is not a unique practice in the gaming industry. However, not all gamers are aware of this. Some users, like ddzed, consider this approach "terrible," and the fact of such early development of the Lord of Hatred DLC "explains a lot" to them. They now link various bugs, shortcomings, and unsuccessful Blizzard decisions to the fact that some resources were simply shifted from the "main" version to the future expansion.