I think Kamelot have simply evolved. They were once a power metal band, with "positive" and "upliftiing" melodies, and lyrical subject often rooted in fantasy. They eventually shifted towards a more symphonic, "goth" and darker approach, which is more aligned with the current tastes of the general public.
I still like them and Tommy is an awesome singer, I consider Haven a fantastic album, but the earlier Khan albums were a bit more "magical" and unique, different from the rest for me. There is a massive load of wonderful songs from The Black Halo onwards, but the Siege Perilous - Fourth Legacy - Karma - Epica string of albums will forever be unparalled to me.