ETF's are so pervasive because they're so easy. VIX futures can be held, but people still buy VIXY and similar products. You can buy literal physical gold, but people still buy GLD et al.

And it hardly matters that wrapping it in an ETF makes it potentially really weird (like how GLD and other gold ETFs are only barely nominally like holding gold, and VIX ETFs often hold cash-settled futures based on the value of a formula that is based on another formula that takes in various parameters of the prices and durations of options which themselves are priced based on the price movements of various equity securities and .........). The depths of weirdness have already been pretty well-explored by existing weird ETFs. A bitcoin ETF wouldn't even be particularly notable levels of weird, imo.

So, just the fact that it makes it really easy is typically a big boon to people getting exposure to whatever financial force the ETF holds.