Because the series connection leads to the drop of the voltage at every loads which are attached in series. This causes divide of voltage over the loads which are connected in series.

But, if the loads are connected in parallel then the current is distributed and voltage remains same which causes the electrical appliances to work well as there is no voltage fluctuation. Also, the parallel configuration is more preferred because in series if there is breakage of connection or if any one of the load disrupts, malfunctions then the continuity of current in the whole circuit is affected and the circuit is opened. Where in parallel connection if the connection is broken off one to two loads the other loads, appliances are not get affected due to them as the current is distributed in each mesh of parallel circuit.