I just find it funny that Iron Lords, supposedly the best of the best couldn't beat Siva. Heck, I find it funny that any of these supposed Fabled Teams of God Like guardians couldn't beat any of the raids either.. But a guardian who has been dead for hundreds of years, who has pretty much no knowledge of anything could beat Atheon, Crota, Oryx and Siva. And literally all we had to do to beat Siva as walk in and press a button. 😕

When? If you're talking about in the past with the Iron Lords, he did and they failed. If you're talking about why he didn't during the events of Rise of Iron there are a number of reasons. First and foremost is the simple fact Saladin doesn't have the authority to muster an army to begin with. He did once but since turning power over to Zavala he has no army to muster. More than that though is the simple fact the Guardians have adopted a strategy that largely avoids large conventional battles for a variety of reasons. For starters there's probably not much more than 10,000 or so guardians in the first place, so when you take into account that many are needed to guard the wall that doesn't leave you with that big of a potential army in the first place. Even if you do muster that army anyways, that's a huge risk you're taking. You've built an army that you must win with because failure would mean such a huge chunk of your overall military gone you're done if this army ever loses. Now lets look at the five major battles we know of where guardians did fight as organized armies. We have Six Fronts, the assault on the Replication Complex, Burning Lake, The Great Disaster, and Twilight Gap. So out of 5 major battles fought we have 3 barely victories and 2 abysmal failures. If we only look at the battles where we are on the offensive our record is even worse, 2 failures and 1 barely victory. One of them is even referred to as the Great Disaster because that's quite literally what it was. Take all that together and you have the fact that fighting a pitched battle where massive army fights massive army is a major gamble we can't afford to lose, and we have a track record saying its very possible we would lose, and even winning may be a Pyrrhic Victory. Which is all why, in my opinion at least, the City has developed and stuck with its current strategy. Use small elite teams in series of low risk high reward surgical strikes to pick apart the enemy's leadership and to secure specific locations of importance.

He was too busy tackling the Crusades.....

The Iron Lords were not trying to destroy Siva to start with. They were going to get release Siva so they could use it to bring back the golden age. One of Rasputin's protocols was to prevent the release of Siva at all costs; this was a higher priority than preserving mankind so he attacked the Iron Lords and killed most of them The few Iron Lords who reached Siva realised it could not be controlled so they tried to destroy it but they realised too late and only Saladin survived Saladin was too embarrassed about the Iron Lords' screw up that's why Siva knowledge is hidden away and he is the only one watching it

Let me answer your question with another question: why didn't they just fly the ring to mordor on one of those giant birds?

The cinematic prelude to roi clearly says "there were no guardians, only iron lords"

Because the Iron Lords were among the first guardians. They didn't have a massive army to use.

He kinda tried with the iron lords. There were like 100 90 of them died in patrol or something, the rest we kinda know about

There weren't any Guardians at the time when they first tried to destroy it. Also, there aren't enough Guardians available to be in an army

He did...they started with 100+ iron lords. Only those few made it all the way to the chamber. That's one of the reasons we only travel in fireteams of 6 max. So we can't all be wiped out at once.

His weapons were nerfed and fired rng bullets. Also the siva lagged everywhere

"Back then there were no guardians, there were only IRON LORDS"-Lord Saladin

Because when people use common sense we don't have any game.