What was the reason for so much volcanic activity on Mars?
Mars was not ever particularly volcanically active, at least not compared to other places in the solar system. The reason that the volcanoes of Mars grow so big is a lack of plate tectonics. Because the plates of the Earth move the magma flow moves as well and do not grow as high, on Mars the magma just kept flowing in the same spot. Here is a link that will explain more than my words can.
