Where are you planning on buying this ETF? I'm guessing it's directly through Vanguard? If so, that's likely your first reason - the majority of brokerage accounts charge a commission per trade for ETFs (and equities) but not for mutual funds.
Another reason is that people who work in the financial industry (brokerages, mutual fund companies, etc) have to request permission for every trade before placing an order. This applies to equities and ETFs but does not apply to mutual funds. It's common for a request to be denied (if the brokerage has inside information due to other business lines they'll block trading, if a mutual fund company is trading the same security they'll block trading, etc) without an explanation. This can happen for months. For these folks it's typically easier to use mutual funds.
So, if someone can open an account with Vanguard and doesn't work in the financial industry then I agree with your premise. The Vanguard Admiral shares have a much lower expense, typically very close to their ETFs.
Source: worked for a brokerage and mutual fund company