AFAIK it’s because some people complained about it. “Rain makes it difficult to see, pls fix” they said. It’s the same story with night. “I can only play at night and my character can’t get a tan” they said, so Blizz removed the weather and night/day cycle. (Yes, it’s still in the game but the nights are so bright I don’t even notice them)
Why Blizzard chose to listen to such complaints is honestly beyond me. But ultimately, most bad changes are rooted in players’ requests I find.