It's the extra layer of convolution. It's necessary in a browser, but when circled back to the server, it's vestigial.
With Deno, you're using a classic OOP inspired superset of JavaScript, transpiling it to JavaScript, and running it in a JavaScript engine on a server. That's only necessary in a browser.
On a server, if you don't want to use JavaScript, you can just directly use a classic OOP language, and dispense with the whole extra frontend transpilation abstraction.
Unless you're a frontend-only webdev, and, understandably enough, you don't want to learn a classic OOP language, which I believe is the real reason.
Or, at least I hope so, because the only other possible reason is mass obliviousness, and that's horrific.
TS hasn't changed the way I write applications too much, besides giving me a bit of safety/enforcement of good practices and productivity boost