Since the OpenSource era beginning, the modern developing environment is built from small and independent pieces. This is our power and our weakness. An average Web app can contain dozens or even hundreds of separate technologies. And this brings the power to replace some parts of it without touching another.
This is especially true for the React related ecosystem: you are supposed to pick every single piece yourself and then build them up. This is hard, but this is powerful.
Say, you are not tied to Babel to compile JSX, you can pick some another transpiler/compiler or even language such as Kotlin. Or you can use no JSX with hyperscript.
But, in Svelte it looks like you don't have the freedom to control every part of your stack. You can't even use TypeScript, which gains popularity every day, and is de facto standard in some companies. There is some work is in progress on it, but is far from finish I'm afraid.
I hope this will change in the future, but today it feels a deal-breaking thing.