They balance Mythic around 20 people, so that is how they make the raid tier. You can raid normal & heroic with 10 people if you like, and mostly succeed. You will struggle on some fights in heroic on occasion due to certain mechanics not being ideal for so few players.
I don't think 10 man mythic raids would be very good. It would be difficult to make them challenging enough. It would also make a few classes completely irrelevant, and I realize that is already the case for some, but it would just further exacerbate this issue.