Armenia had no real modern anti air weapons to combat this. If this were tried on any other modern military those drones would have been shot down at the border.
Iranian drones blew up Saudi processing facilities in 2017; Russian thermite drones exploded two major Ukrainian ammo dumps in 2017. Both Saudi and Ukraine are modern military powers, certainly more so than Armenia.
It's not quite as simple as "shoot it down" - few places have 360-degreee 24/7 coverage of all their borders that is effective (and cost-effective) against both 50,000-foot high altitude UAVs and a swarm of DJI thermite shell drones (not to speak of the semi-truck driving up five miles from your facility and unloading such a swarm.
