- The more people are using (and complaining) the more bugs can be fixed
- The popularity is related with the amount of available documentation, blogs and tutorials
- IDE and tooling support is superb - often there are even several competing IDEs
- Usually all your programming questions can be answered by google
- The probability to find skilled developers is higher, than in the case of a fancy, esoteric language (like e.g. MOO #49 :-).
- You will usually find a variety of popular open source libraries or projects for every possible challenge
- Many commercial providers compete in the same space - your solution becomes less vendor dependent
- You will find a variety of commercial and even free trainings
- The bigger the company, the less developers tend to be interested in bleeding edge fancy stuff
So why not just start with Java - the #1 :-).
NEW: Online Workhop Effective WebApps without Frameworks is also coming to: MUC Airport.