Perhaps it's just Mark's perspective on how the internet is changing the market.
He's like the OG of making money off the internet. I mean he can thank his first breakthrough with selling broadcast.com at 5.7 billion to the World Wide Web. It was really all due to his advantage of creating such a new concept around the time internet first started booming that gave his website so much value.

Yet one big difference (other than bread-winning methods) between Mark Cuban getting money from the internet and other people nowadays that get money from the internet is that Cuban had physical, tangible currency in his hands right away.

Although I also disagree with him calling bitcoin a "bubble", I can see why he's a bit skeptical of the value of bitcoin. He's aware that making money off the internet is not as easy as it used to be, and that if he made a broadcast.com this day in age, it would not of been so successful. Most things have been done, and most success is very brief. Trends are changing everyday. He's said it himself on Shark Tank to an entrepreneur that gave thanks to his success to Facebook, "Facebook giveth, Facebook taketh away." Mark Cuban views on social media contributing to products/ideas popularity and value clearly shows that he sees the internet as a very flexible, unpredictable, uncertain platform. In his eyes, virtual money that he can't pull from a bank right away doesn't really sound all that great.