WHY DID THE LOST PRIMARCHS GET ERASED? WHAT MISTAKE DID THEY MAKE? 

They found Victoria's Secret.

My headcanon is that one of them landed on an alien world where he was raised by a Xenos race and then refused to blindly genocide aliens. Basically a Primarch who was so good and idealistic that it got him killed.

What mistake did the two lost primarchs commit that lead them to be erased? They violated the GW IP policy. There's no coming back from that. ; )

In my head cannon, I imagine that one of the lost Primarchs had his brain usurped/corrupted during the Rangden Genocide. This would explain why the campaign was so hard fought and why records of the genocide were expunged.

Nothing. They were the first two sacrifices to power the Golden Throne. Magnus was going to be the third. That was why Dorn asked Malcador to make him forget, because the truth was too horrific to bear.

I never liked how GW handles the missing primarchs or these two. They treat it as a black box, a writing tool they can fall back on and they just starve us of content.

"...The emperor is willing to overlook things to benefit his crusade...the damage done to Angron"...I should bloody think so! The butchers nails notwithstanding; it was the emperor that damaged Angron the most by whisking him away from his friends instead of saving them as well as Angron!

One of them was Sigmar Heldenhammer, Primarch of the Stormcast Eternals.

What if the emperor did enact a massive cover-up, lying even to his sons about why their brothers were expunged, perhaps even implanting false memories in russ’s mind about the purging of those primarchs. I mean, if two full loyalist legions were to pop up out of nowhere, that would be a massive reinforcement to the imperium. And since each primarch/ legion was designed to fit a specific role, perhaps the lost primarchs were designed as last resort super weapons, and had to be exiled and expunged to protect them from the chaos gods interfering with them, and taking them as pawns.

They beat big E to godhood, obviously.

"What did the lost primarchs do wrong?"

I really love the idea that the 2 lost Primarchs have been battling and stemming the tide of the Tyranids. Them coming back with the stories and information regarding it would be mind blowing. The need for a full scale invasion to be braced against or full scale invasion to be launched into the Tyranid occupied space to stem the tide before it happens would be absolutely amazing.

My headcannon is that one was raised on a planet that revered some hidden exodites and as such was found to be too xeno friendly and escaped into the unknown regions on a Craftworld and passed from communication with the galaxy. The other took shortcuts in the Astartes creation process to create vast numbers which caused the majority of his legion to be taken over by the Rangdan. The second would explain how the Space Wolves had experience killing Astartes and yet it would still be considered unthinkable--they killed Xeno controlled Astartes.

A theory I've heard (though it's old so possibly outdated) is that both II and XI fell during the Rang'Dan xenocide, or some other conflict. What pissed the Big E off (to the despair of the other primarchs) was that His "perfect sons" failed: not failed to conquer their homeworlds in their youth and with no support (see Angron and technically Mortarian), but failed Him after He had found and elevated them to true demigods; even with a full legion of astartes and the might of the Imperium at their back, they simply lost. In those early days, the Emperor felt that He could not show any weakness, so He had the lost legions removed from all records and had their past accomplishments pinned to more successful primarchs. That's why the primarchs who knew them are saddened by their memory, or even angry (like Horus) that his otherwise successful brothers were erased so as not to tarnish the Imperium's image of invincibility.

I like the theory that one Primarch fell in battle during the Ragdan Genocides. The other was purged for failing to come to the other Primarch's aide. A primarch dying would be an affront to the perception of the Great Crusade.

- From the fears of Sanguinius expressed to Horus, we know he was in the same boat with at least one of the two lost Primarch, unless he was referring to the Thousand Sons culling being discussed untill Magnus found a way to stabilize their geneseeds and keep them going.

What if the "Rangdan" were one of the legions that turned against the Emperor , using Xenotech against the Imperium that left the only Legion able to destroy them as the 1st...

My theory is that they mutated to a degree, that couldn‘t be tolerated. You can take that from the conversation between horus and sanguinius. But frankly, we should interview one of the traitor primarchs.

A fall to chaos would make sense when you consider that the Emperor was trying to keep all knowledge of Chaos an absolute secret, especially if only the primarchs fell. Say spawndom even?

The biggest theory I've always entertained is a genetic instability or issue that was exploited by a Xenos race during the great crusade