It's pretty easy to see why ARPANET took a while to develop and turn into the internet we know today. Youtube didn't exist in the 1960's not because people though that streaming video was a dumb idea but rather because it was simply technically impossible to do at that time. In order for the internet to develop it required massive progress in hardware and infrastructure. To this day many places in rich countries still have crappy internet access.

What's Bitcoin's excuse? Where is the technical limitation? Do we need nuclear fusion or 100Terabyte internet links for it to become usable? Do we need high speed data transmission to Mars?

The only thing standing in the way of Bitcoin is people finding compelling use cases for it in order to drive adoption. 10+ years and billions of dollars later it's still only useful as a tool for gambling and scams or to buy drugs and other illegal things online.

Also, ARPANET was already very useful by the mid-1970s (it was declared "operational" in 1975, 9 years after its inception). It was not the internet we know today of course, but it wasn't really meant to be. It was a defence project after all.