It’s a holdover from old BBS forum style.
When you reply to an ancient thread, it immediately gets pushed to the top of the board. Now everyone basically is wondering what this 5 year old topic is doing on the first page. And they might have to read through several pages of messages to understand what the hell the newest reply is about because nobody remembers the topic in the first place.
With Reddit/Lemmy, the upvote system means it really doesn’t matter at all if you reply to an ancient thread, it won’t jump to the front page for anyone.
I like to read in the dark, and a black OLED screen with white text is so much more comfortable than even an e-ink screen for me.
LCDs are good for price, I guess. All my big monitors are LCD, but phone has to be OLED.