The Lorax is that cute, bossy, mustachioed creature who "ga-Zump[s]" (86) out of the first Truffula Tree the Once-ler chops down. We have a couple guesses as to why he's honored in the title.
This is the Once-ler's story, told by the Once-ler and lived by the Once-ler. But, let's be honest, the Lorax steals the show. (And the Once-ler wouldn't have it any other way.) From his walrus mustache, to his tree-stump orations, to his sneezing and wheezing, the cute little guy wins readers' hearts with ease. From a marketing standpoint, he's the obvious choice to promote the book.
Can you imagine the Once-ler on your child's T-shirt or sippy cup? What would a Once-ler action figure even look like? Yikes.
The Lorax also earns named-in-the-title status because of what he stands for: speaking for those who don't have the power of speech (the Truffula Trees, the Brown Bar-ba-loots, the Swamee Swans, the Humming Fish, and probably much more). The Lorax lives to take care of the creatures around him. This means sticking up for others even when it isn't popular and helping others find ways to survive when things get hard.
The whole taking-care-of-your-world stretches across Dr. Seuss's entire body of work. And many more of Seuss's underdog heroes earned named-in-the-title status: Yertle the Turtle, Horton, and of course, the ultimate babysitter, the Cat in the Hat.