Because I don’t want to have sixteen different messaging apps on my phone. While privacy and security are important to me, I’m also only so willing to put up shit user experience.
The reason I downloaded Signal in the first place was specifically because it could do both SMS and encrypted messaging. Then, over the years every new change either removed features I found useful (individual colors for each chat) or added ones that were just a distraction or actively bad (stickers, stories, PIN).
When they got rid of SMS and refused to back down, despite the outcry, I had no reason to keep it.