It's not that they made more money from merchandise, it's that they sold more t-shirts than albums. Implying that more people were interested in the "image" of punk rock than the music.
Gabba gabba hey!
I've also never seen anyone slam dance carrying a Ramones album, but I have seen them slam dance wearing a Ramones t-shirt that got tore up.
Which is a reasonable implication given that punk grew up around the DIY culture. A commercially produced recording doesn't exactly align with the interests of that type of community, even where that community enjoys the music itself.