It's too much to attribute to any one effect. 50% is a lot for a website of this size (don't forget that Lemmy exploded from a migration of <5% Reddit usershare). Let's KISS by attributing likely causes in order of magnitude:

1. ChatGPT became the world's fastest growing website in a single month and it's actually half-decent at being a code tutor
2. ChatGPT bots got unleashed on SO and diluted a lot of SO's comparative advantages
3. Stack Overflow moderators went on strike, which further damaged content quality
4. Structurally speaking, SO is an environment which tends to become more elitist over time. As the userbase becomes progressively more self-selective, the population shrinks.
5. The SO format requires a stream of novel questions, but novel questions generally get rarer over time
6. Developer documentation has generally improved over time. On SO, asking about a well-documented thing is a short-circuit pathway to getting RTFM'd & discussion locked