It's more about Heroku dropping free and low-cost plans, which is them demonstrating that they don't currently care about three low end of the market, more than any specfic feature.
Also the absolute disaster with security they had just before dropping free tiers, and the awful response which took months to even acknowledge some kinds of data (such as pipeline keys) where affected. [0]
> There's no support for a single dedicated IP address for your application. With Heroku, your application's CPU resources are mostly located in one datacenter. Heroku doesn't support HTTP2 or Brotli compression and it doesn't do Edge TLS termination. And it doesn't run your applications on dedicated MicroVMs. These are all things that Fly's Global Application Platform does.
The other comment that mentions Heroku dropping low cost plans is the reason for the explosion in growth as I understand it though.
People don't trust it any more, because Salesforce have been under-investing in it for years and recently rug-pulled on everyone who had ever used the free tier (after providing that free tier for 15 years already - long enough for people to reasonably expect it to continue).