That is just a cost-savings measure as USA does not have a domestic modern naval gun between their 57mm and their 127mm.
This is true - (1)it's sometimes forgotten, but 5"/62 is actually a
very expensive weapon system. You can buy 056a corvette for the price of that gun - and it doesn't even have smart munitions yet.
(2)I suspect they were already ordered when LCS was still expected to be a 50-hull program(or at least such order was promised).
(3)Furthermore, US invests a lot in this gun(MAD FIRES, for example). They like that gun.
(4)For some important functions, it's probably indeed superior(close-in air defense, defeating swarms). If these functions are important enough for the customer - it's a logical choice. Burkes in particular are known to be less than ideal against swarms - so adding constellation to the mix brings a clear improvement.
(5)Finally - USN has more 5" guns afloat than anyone else anyways. If anything - it's in their institutional interest to clearly draw a line between burkes and constellations - or someone unnecessarily smart may ask what they're paying for(Burke).
