Why are wire beaters becoming more common in hand mixers?
Different beaters are used to achieve distinct  effects.  We ask a lot of our electric home mixers: they have to aerate cream, knead dough, mix cakes, among many operations.  Various forces and mixing patterns are needed for these operations.  To aerate air needs to be mixed into a liquid.  A fast moving, half immersed wire forms vortices that mix the air and the liquids.  Thin blades, some parallel and others perpendicular to the beater's motion will break clumps of solids and disperse them among the mixing liquids.  Dough needs to be stretched and folded and for that a single pole into the dough following some planetary motion seems to do the job.

When compared to wire beaters, those with a central post have larger surfaces demanding a stronger motor to rotate them.  A wire beater, while not ideal to mix a cake batter, can get the job done given more time. The central post also collects material, so better designs have been developed for cake mixing.

The type of beaters sold with a mixer are more likely a function of the motor strength and the price category.