Fusion hit a technological barrier that only until recently meant extremely huge hard to build plants to have any hope of positive energy return. Those required many billions of investments just to study physics before any considerations of practical power plants.

Fortunately recent progress allowed newer designs that are much cheaper [1]. That is the reason that we got startups investing in fusion. It is still not pocket money, but at least the scale of investments is 1e8 USD, not 1e10 as before.

[1] https://www.google.no/amp/s/amp.theguardian.com/environment/...