Can some one explain me why all distribution are not focusing in ONE package manager that rule-them-all ?
Why they spend efforts dividing in new package systems instead of easing application spread across all distributions ?
Well, you're bundling all sorts of different issues there.
To take the second one first: the package manager has just about nothing to do with "easing application spread across all distributions", and the package format has very very little to do with it.
If the only differences between distributions were the package format they used, it would be extremely trivial to repackage your 'application' in all the different formats. Someone would invent a simple little tool to do it, because doing it would necessarily be trivial. The package manager wouldn't matter at all, if they were just different tools to interact with the same format(s): if your package was in the correct format(s), it'd work with all the tools.
The true reasons why it can be relatively hard to make an app available across different distributions have nothing much to do with package formats or package managers. It's because the distributions are different in other ways. They include different versions of different shared libraries; some might include a shared library you want to use, while others don't. They all use different desktops - do you write to the legacy freedesktop.org tray icon spec? the GNOME Shell indicator interface? Unity app indicators? etc etc.
I could go on more or less ad infinitum, but I won't. The point is that there are far more significant differences between distributions when it comes to writing software to run on top of them than their packaging formats or package managers. If your primary concern is cross-distro app compatibility, time spent worrying about package formats is pretty much time wasted.