Citadel had nothing to do with the Gamestop meme mania, besides being a market maker that processed a lot of transactions. Anything else you hear is crackpot conspiracy theories with zero evidence.
It'd be like saying "Why is this official with ties to Toyota not recusing themselves, when we already know that the mafia regularly drove cars! How deep does this go?!"
When is it so vital that Yellen, who is an economist and former Fed official and not an expert at all in equity market structure, so vital to be involved in this investigation? What expertise does she have to no one on the SEC staff has?
Market structure is not the subject of the investigation. Market volatility is. In this case microstructure has absolutely zero to do with the volatility. It was caused by 7 million retail traders simultaneously joining a poorly thought out pump and dump. That would create volatility regardless of the mechanics of how orders are crossed.
> Not saying that did happen, not saying it didn't, but Citadel is central
Spoiler: It didn't happen. Robinhood already publicly confirmed that the issue was them running out of collateral at the clearinghouse. Citadel confirmed that it never stopped accepting orders. The DTC confirmed that it raised the margin on GME, causing Robinhood to near its margin calls. JP Morgan and Goldman confirmed that Robinhood drew down its credit lines. Dozens of other brokers, most without ties to Citadel, restricted trading at the same time citing the same clearinghouse issues. The SEC could easily verify all of this. It would necessitate a giant conspiracy involving hundreds of potential whistle-blowers across a dozen companies all risking jail time. And somehow there's zero evidence of any of it.
This has become the stock market equivalent of QAnon at this point.