Because it’s not the public mempool. It’s a private MEV mempool that people pay to add their transactions to for special priority or conditional inclusion. For instance, asshole profiteers can use it to sandwich attack traders to siphon off “market inefficiencies” or some people just want immediate front of the line inclusion in the next block.
Presumably they exploited something in this MEV system (completely unrelated to the Ethereum protocol) that allowed them to see the pool and they shouldn’t have. Wish I knew more but everything I read was incredibly vague and misleading.