This topic is not directly related to CS2D. Moved to Off-Topic.
Also it's "Lua" (big L, small ua, because it's Portuguese for moon and NOT an abbreviation)
How do you define a successful language? Lua is used in tons of apps and games.
It's not only a lot: There are also a lot of big and well known titles in these lists. With LÖVE there's even a game engine which entirely relies on Lua as language.
So in my opinion Lua is quite popular and "successful".
Of course it can't compete with the biggest languages like C(++), C# and Java. That's because it's a scripting language (slower) and I guess because there are less frameworks and less good IDEs for it. Moreover Lua itself does not have classic inheritance/OOP. It's possible with meta table stuff but that's a different approach and people are not used to it which leads to less acceptance.
Also it can't compete with JavaScript because JavaScript runs in every modern browser out of the box and is extremely optimized for that purpose.
I assume that it is one of the most popular scripting languages for games though (if not the most popular one). That's probably because it is easy to use/implement and because the Lua runtime code which needs to be included in projects is tiny, lightweight and doesn't require much memory.
@ script favor: Java is much bigger and takes much more memory than Lua. It would make everything slower (especially loading times). Also it's way more complex and would most likely also lead to more security issues. So this wouldn't be a good idea. Additionally people who do not have the (huge!) JRE installed would have to do so just to run the game. With Lua this is not required because the Lua runtime environment is tiny and fully included.
The only reason why Minecraft is using Java for modding is because it was originally developed in Java. So all the Java stuff needed to be loaded anyway and simply using Java was the easiest approach.
Also I wouldn't say that Java is easier than Lua.
It depends on which language you learned first and used more.
Guess why all these games are using Lua instead of Java...