Because the marksmen would kill the assassin in under 3 seconds.
Now old Leblanc is back, Irelia is better than ever for killing squishies and Rengar Q is back to what it was and I get a feeling that Aatrox is going to become a nightmare to face for many squishier targets. Assassins and dive fighters are getting back to a state where they can more reliably kill squishies.
Now all the marksmen are upset and they are getting blown up .
I can't blame them too much, they can't even build banshee's to stop an ability without getting a lot of useless stats from the item. I remember back in the day if I was playing Leblanc I dreaded the idea of a enemy marksmen getting banshee's because I knew it would take more than one time going in to kill them.
Riot you NEED to revert Banshee's.
And make GA damage neutral in both who it defends against and who benefits the most from getting it. It was a good item when it was MR + armor + revive on death.
Both of these defensive items were good options before they were changed to AD or AP items. The game survived for years with them being options for basically everyone not just AD or AP.
Marksmen have reasons to buy defensive items again but don't have the defensive items as an option anymore. This needs to change.