There's a few reasons. The Fed's mandate to "promote maximum employment" has taken the form of ensuring properly functioning capital markets. ETFs not getting too dislocated from NAV is an important part of that because otherwise they become a useless hedging instrument. Lack of effective hedges increases volatility in markets and forces investors to reduce positions. Further reducing positions during a sell-off causes an even steeper sell-off, etc blah blah.

Additionally and more importantly, primary market transactions are priced off secondary market levels. During the sell-off and before the Fed intervened, corporate bond issuance completely vanished. A large portion of US companies are debt-financed and would be unable to finance operations or be forced to pay way higher rates without functioning debt markets. Ensuring both the primary and secondary market functions properly fulfills (in their theory) their mandate of promoting maximum employment because without either, a lot more companies would have gone under or laid off way more people.