From a tweet on the official Diablo twitter account from March, 2023; we know that the Sorceress and Necromancer were the most played classes in Diablo 4.
The main reason for their popularity is their powerful attacks in lower level, the ability to stay in danger and let pets partially do damage and tank for them so they won’t get hurt. It made combat fairly easy compared to the Rogue, Barbarian and Druid which requires more management during combat and better items to survive better and deal good damage in comparison.
So, the Sorceress and Necromancer have become the preferred choices in the Diablo 4 beta due to their powerful attacks at lower levels and the ability to rely on pets for damage and tanking. This provides players with a more manageable and less risky combat experience compared to other classes. As players eagerly await the release of Diablo 4, it is clear that the Sorceress and Necromancer will continue to be popular choices for those seeking an easier and pet-oriented playstyle in the game.