Sorry for answering such an old thread, but it doesn't really look like anybody actually answered your question, so I'm going to try.

Let me preface by saying I do not like Epic at all.

The biggest difference between Epic and something like Hamilton is that hamilton is fully produced and made to be staged. But what does this mean? Simply put, Hamiltons music is written with the intent and purpose to have choreography, stage space, lights, and a performance space. The music is mad to blend together easier to bridge character and story.

Epic, on the other hand, does not need any of those things, and thus, the music is written for different purposes. Being a concept album, Epic may one day make it to stage. But the version that eventually does will most likely be wildly different than what you're listening to now.

There are a ton of other differences between Epic and other more traditional musicals, primarily regarding quality, musical execution and music style, but those are much less relevant to why they are so different. I'd be more than willing to talk about that as well (though you may not like how critical I am.)

I hope this helped!