Because because Lyor and Russ have both gone on record numerous times saying that signing Redman saved the label. They have said since then even though there were rough patches, they have never looked back. He was fresh blood, new life for the label.
But if you really want to get techincal, you can say the Polygram saved the label. Def Jam has always had platinum sellers, but you can say that Sony's ownership was the reason for their financial troubles, so Polygram came and brought them under the umbrella. "My boy MC Serch nevertheless, Took me to Columbia, back then CBS. Chris Schwartz, RuffHouse, he was the best man, Now bugging because the label had just dropped Def Jam. Could you picture Russell needing a check, man? But he smart, he plotted a plan for Polygram". I don't remember exactly when Violator came under Def Jam, but that was around the same time, cause I believe Warren G was Violator at that time as well. Honestly, even with that track being everywhere, I didn't even know that Warren G went triple. I thought it was just the single that was big and his album went gold to 1x Plat like everyone else. Regulate was really considered wack to OK, here. I need to see dates that it reached triple. No one in Philly was talking about Warren G or that album.
Bottom line, y'all are making it like Warren G single handedly selling whatever numbers he sold saved the label, when they have always did well in selling music. They stopped being in red when they adjusted the formula and also started in dealing with being a distribution company as well (Roc, Violator etc), eliminating/reducing costs. And even then, Meth came in November and basically did the same for them that "Warren G did"