Why is the word “the” declining in English?

The drop is indeed puzzling, but unlike Brian Collins I don’t think it reflects an actual change in English usage (such as the perishing of the encyclopaedic the—that wouldn’t make that much of a dint). I also don’t think Second Language Learner English would make such a dint. It’s about the representation of texts in the Google n-gram corpus.

The dip seems to be from 1960 on. What I think is happening is more texts from then on in the Google corpus have headline-like English, of the kind you see in dot points; headline-like English drops the readily. It’s a real written register (in business reports, instructional manuals, &c), and it’s not one that was really around before then. So:

• “Attach nozzle to faucet, see diagram A”
• “Referred director to board for more information”
• “Computer incapable of processing input”

EDIT: See comments: this is unlikely to account for the drop either, especially in fiction.