We have restrictions on HTML5 game uploads to help create a good experience for people running projects in their browser.
You can learn more on our documentation here: https://itch.io/docs/creators/html5#common-pitfalls
Here’s the relevant section:
Uploading a very large game — Although many game engines allow you to export anything as an HTML5 project, often you need to consider what the user experience of playing the game will be like. For larger games, it’s often better to go for a downloadable version. This is why we have default limitations for project sizes (although you can contact us if you need lifted limits). Projects can still be easy to play with the itch.io app. You can think of an HTML5 project like a website: many of the same performance optimization recommendations still apply.
Because your game is so large, we highly recommend creating a downloadable version to ensure the best experience for your players.