When one thinks of daggers the "stiletto" style of blade is the first thing to come to mind whereas a dagger has a broader blade sharpened on both sides which can be used as a slashing weapon. A dagger is a somewhat specialized piece which can serve as a knife though it isn't as handy. If the spine is reduced in thickness and the point somewhat blunted the dagger becomes a double edged knife and is much more practical in this form. Eliminating the guard or reducing it in size makes any knife far easier to use for most chores except fighting. A double edged knife offers the advantage of not needing to be sharpened so often but requires twice as long to sharpen when it needs to be. They are also somewhat of a compromise as the blade can't be shaped to perform some tasks easier. All that said double edged knives were not uncommon, just not predominant.