Occam's Razor, I suspect X AI simply downloaded/scraped all porn they possibly could access on the internet to build their training data, and this included a large amount of child porn.

Likely Elon-style factors:

• time pressure. performed in a huge rush (he joined the LLM party late with massive FOMO),
• zero internal pushback. Majority highly pressured immigrant/temporary staff (H1-B visas used a lot by Twitter in wake of Musk buyout)
• zero importance placed on ethics / content review by leadership, likely even an ethics-negative internal culture. "Move fast, break things, fuck the government regs".
• loathe to pay licensing fees for materials or 'waste time' negotiating content-access contracts from legitimate porn businesses ("just scrape all the torrents and free sites you can find, the internet is full of free porn").