Probably no technical limitations. Just impractical.
Pretty sure many types of jetpacks have been made. It just hasn't caught on.
http://en.wikipedia.org/wiki/File:Rocket_man02_-_melbourne_s...
Pretty sure many types of jetpacks have been made. It just hasn't caught on.
http://en.wikipedia.org/wiki/File:Rocket_man02_-_melbourne_s...
There are also strong constraints on the altitude and maneuverability (between physical/design constraints and the increasing likelihood that you'll lose control).