It underwent a name change without really being a different project
The name difference was only to differentiate the legacy .NET Framework with the new .NET Core while both were being developed concurrently. They never intended to keep the “Core” suffix forever. .NET Core had a lot of missing APIs compared to .NET Framework 4.5., and “.NET 1.0” would have been ambiguous. It was to signify that it was a new API that isn’t fully compatible yet.
Once .NET Core implemented nearly all the APIs from the legacy .NET Framework, the version numbers were no longer ambiguous (starting from .NET 5.0), and the legacy framework wasn’t used as much as it used to be, it made sense to drop the “Core” suffix :)