Why did this stop being a thing?

Not unicycling - but I just saw this shared to the unicycle chat Facebook page and if Roger at unicycle.com wants a new product line, he has at least one customer!
The Birth of Rollerblading: Cycle-Skating (1923) | Sporting History - YouTube

And just to drag us back on topic…

It was probably a lot easier and cheaper to just buy roller skates. Probably a lot easier on the ankles too.

You couldn’t do the off-road with roller skates though - theoretically, those wheels could even be a bit bigger. Stopping would be a challenge though.

That’s true. It’s interesting. I guess it just never caught on.

Can too.

Ok - but that’s two bikes rather than two unicycles!