Because investors like hedge funds aren't setup to take custody of Bitcoin. ETF certificates, doesn't matter if they are for Bitcoin or frozen concentrated orange juice or Vanguard Trust are all just products that can be traded easily. Existing stock and commodities exchanges can facilitate the trading of them without having to do anything.

And traders aren't interested in investing in slow moving long term blue chip stocks. Traders want volitile products that fluctuate by large amounts daily. Traders don't care about the underlying asset. To traders, Bitcoin ETFs are nothing more than casino chips.

The only thing that matters is that the ETF fund has the Bitcoin they claim to have. Because anyone can verify the Bitcoin held by the fund, it makes Bitcoin ETF funds very trustworthy. Other types of ETF funds can easily fake their holdings.