(The article touches on this bit a little) I was watching something about fusion the other day and it seems that it is super tricky to keep the magnetic field balanced in a way that keeps the plasma in a proper toroid. Not only does it need to keep the correct strength, it has to fight against random turbulence. This is critical to start the reaction, but also to maintain it.

Also, they gave some other physical limitations in the article as well:

To extend their plasma's burning time from the previous record-breaking run, the scientists tweaked aspects of their reactor's design, including replacing carbon with tungsten to improve the efficiency of the tokamak’s "divertors," which extract heat and ash from the reactor.

Basically, it's the container that has limitations as containing a pseudo-sun probably isn't easy.