Debian isn't known for the latest and greatest. They use what they determine to be stable for a given release and that's often a little bit older.

So it means they are waiting for more bug fixes to upload 4.18.x? Then how are some releases uploaded to experimental?

my understanding is that Debian/Ubuntu preference ABI stability within a release - and that ABI stability is not guaranteed going between kernel branches. I believe this is also the case with CentOS/RHEL too.

I might be totally wrong about this.

I don't know specifically about why 4.18.x hasn't entered sid (unstable) yet but generally this means that uploading to sid may break other packages in some way, or the new version has something new that is still not packaged, integrated and tested properly with the rest of unstable packages.