Without bare module specifiers (like `import {html} from 'lit-html'`) to act as an abstraction over location you have to import directly from a URL, but what URL? Packages either have to be self-hosted, or very often npm packages are loaded via CDNs. There are several CDNs that host the same packages (unpkg, Skypack, jspm, jsDelivr/esm.run) so there's no single canonical URL to load anything from and each module that loads a package could load from a different CDN, or simply load a slightly different but compatible version, causing multiple copies of the packages to be loaded.
There's a huge benefit to abstracting over package names, and Deno mostly throws it all away. It does support import maps, but you'd need tooling that doesn't fully exist yet to crawl and dedupe a module graph and at that point it'd basically be npm but a lot slower with no registry to use for metadata.