Because V8's low-overhead tenant isolation is fundamental to how Cloudflare Workers works[1], and without it it would be a completely different product with worse performance. This means it can only support JavaScript and WebAssembly.
You can run it locally[2], and efforts are underway to standardize much of it so that the same code can run on Workers, Node.js, and Deno[3]. The Workers runtime is also open source[4], although in practice this is likely to be less useful for most use cases than just using one of the other JavaScript runtimes if you're self-hosting.