The Great Planes Spacewalker ARF could have been discontinued for any number of reasons. Great Planes essentially owns Tower Hobbies more or less, so if Great Planes makes or distributes something, Tower Hobbies sells it. That particular ARF may have been lagging in sales compared to other, more aerobatic .60 scale ARFs, or perhaps Great Planes is having supply issues with an overseas partner. Great Planes could simply be preparing a new version of this ARF that isn't ready to ship yet. You might try calling them and asking if you're really curious.
Great Planes isn't the only ARF distributor with a Spacewalker ARF model. Seagull makes a .40 sized Spacewalker and Spacewalker II ARF. Seagull is distributed in the U.S. by Horizon Hobbies:Horizon only carries the Spacewalker II .40 ARF, but it seems to be an attractive plane. The "II" refers to the double cockpit on the model.
There are a lot of ARF planes available as .40 or .60 scale craft similar to the Spacewalker/Spacewalker II that you might consider. What makes the Spacewalker a good second airplane is the low symetrical wing for good aerobatics combined with average control surface areas and a large wing area for stable, gentle flying characteristics when flying level or landing. These characteristics also describe the following ARFs to a great degree. If you don't like the Seagull Spacewalker II .40 ARF, perhaps one of these will be more to your liking:
Great Planes Easy Sport MkII .40 ARF Great Planes Big Stik .40/.60 ARF Great Planes Super Sport .40 ARF Great Planes Rapture .40 ARF Great Planes Slow Poke Sport .40 ARF Great Planes Lancair ES .60 ARF Tower Hobbies Kaos/Voyager .40 ARF Carl Goldberg Skylark MkII .40 ARF Carl Goldberg Tiger 2 .40 ARF/Tiger .60 ARF SIG Four Star .40/.60 ARF Hangar 9 SuperStik .40/.60 ARF Hangar 9 UltraStik .40/.60 ARF Hangar 9 Cherokee .40 ARF Hangar 9 T-34 Mentor .40 ARF Seagull PC-9 .40 ARF Seagull Decathalon .40 ARF Super Flying Models Eindecker .46 ARF Thunder Tiger TigerStik .40 ARF
All of the above ARFs are available at either Tower Hobbies or at Horizon Hobby. This list is by no means comprehensive, and I may have left out some other suitable second planes. Hopefully you can find something else that you like as well as the Spacewalker.