Nah, it’s not 1% of battlegrounds, definitely under 10% though due to the rush strats that are preferred by random PUGs. Most AV’s simply don’t last long enough, so successful summons are infrequent.
It’s not as “rare” as you think. In fact, you could summon Ivus every single game if you wanted to - it’s just that bg casuals are lazy and/or don’t turn in their items. It’s the same thing on Horde, I remember back in the summer I had to pretty much beg PUGs to come and help summon Ice Lord or click on the altar.
I made a thread dedicated specifically to these bosses a while back (on the old forums?), but here is the TL;DR “process”:
STEP 1: collect 200 “Storm Crystal” for Ivus / collect 200 “Stormpike Soldier’s Blood” for Ice Lord, these items are looted from the corpses of dead enemy players (in other words, you have to PvP to get them)
STEP 2: actually turn in these items at the Alliance / Horde base respectively (most important part!)
STEP 3: have ~10 peeps ready in the middle of the map for when the Ivus summoners / Ice Lord summoners arrive, time is of the essence since the summoner NPCs can sometimes bug out if attacked by enemy players - it’s best to be quick
STEP 4: have all 10 players click on the summoning thingie and don’t move, the process seems to take about ~15 seconds so ideally mid should be clear of enemy players when you attempt this
That’s really all there is to it, it’s not really “complicated” it’s just that casuals are lazy and either don’t hand in their items or don’t pay attention to the chat.