Why on earth just one gun mount? Thought by now someone would have figured out its the weak link.
That's the reason. Medium calibre guns are petty low on priority these days. Too short ranged for SuW or AAW, the only use is shore bombardment. Which is why every warship designed by anyone in the last 30 years has kept one for backup (and tradition), but they are no longer the primary weapon system.
The gun is a backup. This ship, like every other surface warship these days is primarily a missile combatant and indeed it has eight VLS missile cells that it can ripple fire. The gun’s use (as noted by the above poster) is for shore bombardment and engagement of surface or air targets not worth expending a missile on (think Somali pirates in speedboats or similar.)