Anecdotally, ITER was the largest of few options for a fusion researcher to run their experiment in a new tokamak. Everybody wanted to put their work into it, and as more features were added, the more funding it sucked up, leaving less money for other experiments, leading to more people wanting to put their experiment into ITER. Here's a presentation [0] that goes over why SPARC, being so much smaller and simpler than ITER could be more likely to succeed.

[0] https://library.psfc.mit.edu/catalog/online_pubs/iap/iap2016...

This quote from the presentation summarizes it well:

“The more money that's involved, the less risk people want to take. The less risk people want to take, the more they put into their designs, to make sure their subsystem is super-reliable. The more things they put in, the more expensive the project gets. The more expensive it gets, the more instruments the scientists want to add, because the cost is getting so high that they're afraid there won't be another opportunity later on- they figure this is the last train out of town. So little by little, the spacecraft becomes gilded. And you have these bad dreams about a spacecraft so bulky and so heavy it won't get off the ground- never mind the overblown cost.”

“That boils down to the higher the cost, the more you want to protect your investment, so the more money you put into lowering your risk. It becomes a vicious cycle.” - Rob Manning, Chief spacecraft engineer, JPL