Oscillating currents tend to release radiation only perpendicular to the direction of the current itself. A dipole antenna is maybe the simplest antenna to demonstrate this. There isn't really a good reason why, other than this result pops out from the math. The best explanation I can think of is that currents only tend to create magnetic fields perpendicular to the direction of the current. This is also true in electrostatics, when you study the Biot-Savart law. It's also related to how the magnetic field is given by the curl of the magnetic vector potential, which points in the same direction as the current.
There will actually be fields in the vertical direction, but only non-radiating electric fields, that are closely bound to the antenna.