It is an arbitrary limit. And there are like three of them. Unzipped file size. Unzipped individual file size. Number of files.
I do not really understand the limit, since a browser will probably download the 27MB zipped into cache anyways. In theory only the files needed would be transferred and loading each tiny piece of texture individually would create an overhead. But as I said, the limit is arbitrary. Apparantly the theory is, that engines that export with so many files are better suited for having downloadable games and using the engine export as a html5 game lessens the user experience, so it is discouraged by that arbitrary limit: read that as, Itch would rather want you to have a downloadable game, if it has to have so many small files.