The Super Robot Wars series was born from the most basic mindset of any fanboy: "What if my favorite characters from my favorite series met up?" The original game, Super Robot Wars, was released on the Gameboy in 1991 and was a tactical RPG featuring robots from (essentially) 3 different series: Gundam, Mazinger Z, and Getter Robo. From there, the game spawned multiple sequels and spin-offs bringing in different robots and characters from different anime and games together to form a sort of robotic Justice League. If you can think of a popular Japanese anime featuring robots, there's a good chance it shows up in one of the games. Even the robots from Virtual-On made an appearance.
The stories were an amalgamated mess of all the different series involved. The developers at Banpresto did some creative weaving to put together plots from radically different stories, often taking some license and slightly altering things so they would fit. They also played towards fans by allowing things that never happened in original series to happen, like having popular characters who died, survive, or characters who were originally enemies, team up.
Along the way, Banpresto introduced some of their own original characters and robots to allow players to spice things up and allow players to feel more involved with their favorite stories. In fact, they created so many, that they were eventually able to create spin-off games and anime series featuring only their original characters.
The unique nature of the series has proved to be quite successful and allowed it to continue spawning newer sequels for over 20 years. The series has also gained numerous fans outside of Japan, many who yearn for a localized version, or perhaps their own game featuring their favorite characters from their own favorite series.
Sadly, due to the fact that most of the series that appear in the game are owned by different companies, all of which Banpresto had to obtain consent from to use their IP, simply making a single game is a legal nightmare. Considering the costs and labor to translate a game, plus the low potential for sales numbers, localizing one just isn't worth the hassle. If even one company says "no" for any reason, then the entire game is gone.
In regards to a different game utilizing the Super Robot Wars formula, while many Japanese companies gladly give Banpresto their blessing due to the popularity of the series and the money it makes, outside of Japan, few would give anyone the freedom to do the kinds things Banpresto does. In fact, in most cases when different companies share characters in the same feature, oversight from both sides is rigidly strict to almost ridiculous levels. In the movie Who Framed Roger Rabbit, there's a reason that Buggs Bunny and Mickey Mouse have the exact same number of words to their dialogue. In a Marvel DC crossover, there's a reason that characters on either side will give and take the same number of hits in a fight. At present, the closest thing anyone will have to a Super Robot Wars-like game outside of Japan is the Kingdom Hearts series. And that's a joint between only 2 companies.
The Super Robot Wars series will enter its 21st year on the 20th of April, and it looks like it will remain a strictly Japanese game for the foreseeable future.