It's about the amount of power needed to run all the functions actually. Soon things will change with more powerful processors that are energy saving and larger batteries.
i did the whole BB thing when i was traveling heavy for work and what not so it came in handy for corp email but nowadays you can get a great non BB phone and still get push email service for cheaper. plus BB's are far too propietary with the apps and software they support.
if i were you i would consider a Nokia E series for a nice business phone but with the media and other perks of say an iPhone or SE.
and don't go with WM, just like its desktop counterpart its just a piece of shit