Because after 10 years they see some rough patches and want to update them, while still using the same core rule and being compatible with all of the 5e books and adventures they have already released. D&D naming conventions have never been consistant for 50 years. What ever name they choose will cause some confusion, because they have never used the same system twice.
Honestly just sticking with 5e is probably going to cause the least amount of confusion of any name they could choose. Calling it 5.5, or 6e, or what ever else they could come up with, would actually cause more confusion and lead people to believe that the old 5e adventures are not compatible with the new version.