Likely because it follows the highly organized release cycle Firefox has adopted a couple of years ago. If the new pdf.js is included right now in Nightly, which is necessary in order to verify its integration in the browser, its interaction with other elements and to ensure a long enough test phase on rendering before it lands on release channel… well, if it is right now in Nightly, let us do the math:

* Firefox (release channel) is now 30 * Beta is 31 * Aurora is 32 * Nightly is 33

That's why.