Let’s stop kidding ourselves. They made it simply because they wanted to make some new crap for the marketing purpose (despite being logically redundant).
Well, you could make that same argument for a lot of the OT Stormtrooper variants.
I mean if a regular Stormtrooper with a few add-ons could function in the desert, then why did we get a completely new type of trooper for forests?
It’s a lot easier to sell one of these rather than just changing the regular Stormtrooper a little bit like they did in the first movie before they knew it would sell ridiculously well.
Also, do we really know that the Shoretroopers are infantry? None of us has seen the film yet, and they even look like Scout Troopers. They could literally just be a tropical variant of scout troopers.
(Also, do we even know yet if “Shore trooper” is the official name?)