Well, the build didn’t look like the quality was great. But the biggest thing they should’ve done is put the A7 in it.
The 5c was supposed to be a iPhone 5 that was cheaper and easier to manufacture for Apple. Not sure why you think it deserved the A7 then.
Though Apple clearly learnt an important lesson from the 5c fiasco - which was that Apple customers want the latest and greatest (or at least, what was once the latest and greatest), and don’t mind paying for them. That’s why the 5c failed, because users didn’t want to be seen with what was clearly the inferior budget option.
Hence the Xr getting the latest processor and being given equal billing and marketing.