Why did credit card computers not take off?

I noticed a Rex device on eBay the other day and was sorely tempted. In the end I resisted though. Which is probably good. But it got me thinking about these kinds of devices. I’ve posted about these before, a number of times and there is a page dedicated to all things PDA as well. I wonder why credit card sized devices never took off.

I know that smartphones have taken over almost every use case of mobile computing, but I do think that the result of that is that everything is a little dull. Sure we have smartwatches and phones, but the lack of a variety of form factors makes everything a little too uniform in many ways.

Something to think about maybe.