For Java, there is a standard mechanism to just add the jar file. But for Mathematica, there is no standard way to embed a sub-package into a main package. For LTemplate it wasn't trivial (and I would not expect someone new to packages to be able to do it).
With Java, applications are isolated and do not interfere, even if they run on the same system. With Mathematica, "the kernel is the OS". All loaded packages share the same system. There is a much higher chance of package conflict / version conflict.