They killed sales with the aluminum fork. A steel fork was cheaper(?) and better.
The 520 was my second touring bike, and quite possibly the best one I have had (although my Soma Saga disk is better, I spent far more money pimping out a steel frame and fork -- you couldn't get the whole bike).
My first significant tour with the (1990) 520 was down the West Coast. It held up great. The headset got slightly loose, but apart from that it was flawless and zero maintenance. I finally gave it away (ca 2005) after years of heavy use, and it was still a great bike then.
I guess they just didn't make enough money on it, and it became harder to source the steel frame and parts during the 2020s.