See the change can be contributed to several key factors.
The first is, TIME, it has been several years since the end of Halo 3, the UNSC would be researching better tech for there spartans, thus the new designs.
The second is that these are Spartan 4's, these new generation of Spartans would require different armour for post war situations.
Thirdly, this new Halo is being developed by 343 not Bungie. 343 would be trying to stamp there identity into these new Halo's, while keeping a familiar feeling.
Personally i love these new Armour Designs. It looks more functional then previous armours.