Why design something new when you have something that already works?
Lots of reasons. Look at the Rebels TV show. There was no need to invent new ships, but they did.
When you give artists a chance to create new Star Wars designs, they will come up with stuff so freakin' cool you'd be crazy not to put it on screen.
New stories demand new stuff, sometimes just to give us eye candy, sometimes because it fills a (specific) role in the story, sometimes both.
New stuff equals new merchandise.
There are also lots of reasons to use familiar designs, and I think we'll see lots of them, too.
Edited by DagobahDave