Almost every band at some point try to change sound and to be honest the main reason is to gain new fans and earn more money.
I belive it is very frustrating for bands when you are recording album after album, working so hard then some Nirvana or Guns came and after one album become "bigger" than you.
So eveybody try to adopted to more American style becuse if you are big in America that means more money for you. Maiden, Metallica, Helloween, Sabbath, Ozzy, they all try at some point gain more American fans.
I dont know why but any type of metal with symphony or keyboards is not very popular in USA, so normal thing is that SIT and 7th never got big success in USA.