Speed, mostly. There are blowers that also act as vacuums. It works okay, but still pretty slow, if you blow it all into a pile first. You could do better with a wide device that worked like a lawn mower, but that would only be useful for a fraction of the areas you are trying to clean up.
they exist, I have a one that works both ways, it can blow leaves and it can suck in and mulch the leaves into a bag. Some reasons you don't see it as much include
1. mine jams extremely easily when sticks or wet leaves get into it
2. their ability to actually move leaves is significantly less when in suck mode vs blow mode, this is not specific to moving leaves, try sucking sugar throw a straw vs blowing it, blowing moves stuff MUCH more.