UK plugs (and some others) have partially sleeved line and neutral pins to ensure that the metal cannot be touched when the plug is partially engaged. Other plugs, such as the German Schuko, rely on the plug being inserted into a recessed socket. (See picture, far right, of a Schuko plug in its recessed socket, note the side earthing contact in the upper socket.) As universal sockets can have no recess there is nothing to prevent touching an unsleeved line pin on a partially engaged plug, as demonstrated by this Schuko plug in the Lengon socket.
A fundamental reason for not using a single plug type is that there are two very different mains voltages used in the world, the European voltage is a nominal 230 V, the American voltage is a nominal 120 V, other countries use one or other of these. International standard IEC 60884-1 requires that a plug intended for one of these voltages may not fit into a socket intended for the other. Universal sockets do not comply with that standard. Plugging a 120 V appliance into a 230 V socket may result in a fire!
Since its invention, the international community has adopted the Wonpro Universal Socket as the de facto standard power adapter to connect electrical appliances from any country to the wall outlet in another host country. This is the most effective way to get connected & get power in a foreign country.