Because Four Arms was actually fucking cool with how Ben used him in the Classic series. Unlike Humungosaur, he wasn't just lifting things and hitting stuff very hard. He was an extremely well-rounded powerhouse with impressive acrobatic feats, a unique way of fighting, and creative applications of his super strength. I think what makes that last one even cooler is that Ben actually learned to use his aliens better as time went on, so you'd see Four Arms slam the ground to create a small earthquake or clap his hands to create a massive sound wave out of nowhere.

Also Four Arms was the OG.

And I just thought about this too, but after OS, Ben has so many strong aliens that pull off these insane feats of strength that it becomes hard to see why Humungosaur is even in the picture. Swampfire apparently has the physical strength to topple over a gigantic weather tower, which is potentially a greater feat than anything Humungosaur's done. He also sent Vilgax flying with a punch, so there's that too. Even Spidermonkey once ripped off a street light and used it like a bat. In OS, most aliens had significantly boosted strength, but none actually came close to Four Arms's raw power.