The planetary scopes tend to be longer FL scopes as you need to build magnification to see the planets. You're pretty much limited to 50x/in. of magnification so your scope will easily achieve 400x. You can build a scope with a long FL and get the high magnification with a nice EP while a shorter scope will be needing a much shorter FL EP to get the same magnificaion.
On the other hand, many of the deep sky objects out there are on the order of a degree in diameter or larger and thus you need to have a shorter FL scope so you can see all of the object.
Simple geometric math on the central ray will show that as the FL grows, the EP needed to see something grows larger in diameter as well as a larger FL for the EP to maintain the magnification.