The reason, specifically, is because composer scenes are essentially tied to a single module (Lua file) which means they are bound by the same loading behaviour as Lua modules are - that is: once a module is loaded, calling 'require' to load it again simply returns the same object as calling it the first time. You need to deliberately tell Corona to remove it if you want it to be loaded again.
Yes, it is a bit irritating when you want to use a single scene module and have it dynamically load your level data, etc.
Please, take the learning from others here and just accept that, at least in the Lua world, this is more of a beneficial side effect that a negative - in general.
To achieve, at least, the effect of one scene reloading there are a number of things you can do...
Create a function which returns a display group which can behave in the way you want your games conceptual scene to behave (fading in/out, etc) and use that within your composer scene. Rather than using gotoScene to leave the scene, simply create a new game level object and transition it on while the old one is transitioned away. This is good code reuse and gives you great control. (I made the beneficial statement above because, as you code this solution up, you'll see that there are many clever things you'll want to do which composer scenes are also not the right solution for - such as complex effects or anything which is specific to your app and not generic.)
You can take the easy route and simply goto another 'intermediate' scene, unload the game level scene and then goto the game level scene again. This will cause it to reload. It doesn't help if you want them both on the screen at the same time.
You could also goto an intermediate scene and then goto the game level scene again, but pass it a parameter which causes it to load a different game level during the 'show' 'began' phase. This might suffer from stutter if you have a lot of loading to do.
There are probably lots of other solutions, but these are the ones I tend to go for. (I'm not a wizard.)