If you read the official Moselsteig website, all it says is this:
Planning started in 2009, and the Moselsteig opened in April 2014.
They also note that funding was provided by the EU as well as by the State of Rhineland-Palatinate, but that's it - they're pretty brief about the whole thing.
Now, from my North American perspective, the whole thing is a lot more interesting than that. Even though I lived in Germany for two years (junior year in high school, plus junior year in college), I wasn't even remotely aware of the Germans' love of hiking. (That's probably because I was more interested in typical adolescent stuff, like drinking beer and losing my virginity. You know, the usual.)
There have been German hikers' clubs for nearly two centuries at this point; they sprang up in the latter half of the 19th century and are still pretty dang popular. The oldest and the biggest ones in the country are in the southwest corner of Germany - Baden and Swabia, respectively - but the most relevant one here is the Hunsrück club, founded in 1890. Now, if you're like me, you probably just thought, um, wait. The Hunsrück? What's that?
It's a mountain range. The Hunsrück runs roughly down the southeast side of the Mosel. It's more or less mirrored by another range on the northwest side, the Eifel. Now, as a Californian, I feel compelled to point out that these are mountain ranges mostly in the sense that they aren't plains; they're not particularly tall compared to the Alps or the Sierra Nevada. Heck, here in San Diego, our tallest mountain is two and a half times taller than anything in the Eifel or the Hunsrück, which means that if you're looking for challenging Alpine hiking, well, the Moselsteig ain't it. As a wine drinker, though, I should also point out that San Diego's very best wines are distinctly meh compared to Mosel wine, though, so there are of course a lot of other reasons to visit the Mosel, like fairytale castles, friendly locals, delicious food, and all of that. But I digress.
Around the turn of the 20th century, the Hunsrück Club started mapping out the Moselhöhenweg, or the Mosel High Path. The MHP ran along both sides of the river - after all, the idea was a High Path, or a path that followed the hills or mountains along either side of the river and not the river itself. It started at either the Luxembourgeois border (for the Eifel side) or the French border (for the Hunsrück side) and continued all the way down to Koblenz, which is where the Mosel river empties into the Rhine and where the two separate paths met up with each other. And that was that.
Fast forward to the 21st century, and I'm sure that local businesses and tourism authorities thought hey, you know what? Why don't we band together, get some of that sweet EU money, and come up with an awesome new trail with killer branding that combines all of the best bits of BOTH of those trails, improve a lot of the existing trails, slap new signs on everything, and call it Moselsteig? Maybe even translate a marketing brochure into Dutch - maybe even English, why not? - and see if we can drum up some interest? Maybe even enough interest to get a shout-out in the New York Times and if we're really, really lucky maybe even some Californian guy named Chris Pratt to come visit? Well, it's obviously working 'cuz I leave in five weeks.
One final note for now, and that's about the name of the trail itself. There are a fair number of long distance trails in Germany with the name Steig in them, which I first took to mean "climb" as it's obviously descended from the German verb steigen, "to climb." However, I didn't realize that Steig has long since come to mean something simpler: a path (usually one that does have uphill sections or is in the mountains). I guess I should've known that because there are other German words with the word steig in them that I was familiar with, like Bürgersteig "sidewalk" (which always sounded weird to me - it sounds like "citizens' climb") or Bahnsteig "platform" (as in train station; Bahn meaning railway), both of which suggest places that you can walk that are next to places you can't like roadways or train tracks.