Pascal didn't make dramatic changes over Maxwell in the same way Maxwell did from Kepler. There are changes, but for average workloads it was just faster Maxwell (thanks to the process node change). This meant that both from Nvidia's side, driver updates and such would tend to still benefit Maxwell a lot, and from Developers, catering to Pascal also meant you'd probably have good or at least decent performance on Maxwell and vice versa.
The other thing is that Maxwell was itself more of a forward looking architecutre than Kepler. It was a better, efficient arch. Async compute wasn't a huge deal in the end so Maxwell (and to a lesser extend Pascal) being lacking there didn't matter much.
Combined, this meant that Maxwell has remained very viable for a long time. It's effectively the exact same recipe that resulted in AMD's "aging like fine wine" (unless you're a Fury card lolol) meme when Maxwell first came out. GCN revisions weren't major changes, and they had forward looking feature sets. Ergo, the cards lasted a long time.
Remember when "Nvidia is sabotaging old card performance to force you to upgrade" was a meme? Lol.