Probably because pdf.js is implemented using <canvas> so it's basically like printing screenshots of the PDF
Canvas is just a bitmap so when printing pdf.js renders to a certain dpi canvas which I believe is less than 300 dpi (150?) which even still uses huge memory and ends up with fuzzy text.
You don't want bitmaps going to printer for text and line art, you want vector so it can come out at 600+ dpi while using minimal memory.
This is what seems weird. Why is that? Also it's not like most printer drivers don't need to convert it back to ps or pdf before printing.
SVG is turned into the appropriate PS/PDF vector drawing command by the browser print engine, canvas just gets sent as a bitmap in the printer language since that all that's left.
I believe pdf.js incorporated and modified https://github.com/gliffy/canvas2svg which implements the canvas api but instead creates an SVG dom.
Printing is a forgotten corner on the web, would be nice if a mainstream browser implemented the full css print spec too so we could create page perfect output without relying on PrinceXml...