For the same reason you can’t call arty every 30 seconds like it was at the beginning - to make artillery less powerful.
In this case you really should listen to “us old farts” - artillery is pretty well balanced right now.
Because nobody uses smokes since it doesn’t give you instant score
Neither does artillery half the time.
But that’s not a reason to allow it to be spammed.
It won’t get spammed since only one person will able to call it every few minutes. It could get even higher cd than the arty across the team.
God can you possibly be more naive??!!
Every player with a “free” artillery call will be able to call it at the same time if they want!!
They can’t since they have to wait several seconds when it stops firing and not everyone is using radioman at the same time so you rarely see arty spam after the first point has been captured.
My radioman is usually dead before I can use him anyway since everyone on the team is trying to call artillery at the same time. I could hide him 50 miles beyond the grey zone and somehow he would still find a way to die. 
Maybe it’s the game’s way of thinning out the artillery calls. IDK But its frustrating. 