I think this is more of a geological-sculputural question not historical, but lucky for you I'm a geoscientist.

Granite is a deep, igneous rock rich in quartz and feldspars. Marble is metamorphised rock made up by carbonate minerals like calcite and dolomite. Visually granite has a coarse grainy texture with visible crystals. It tends to be uniform because granite typically lacks any internal structure. Marble's appearance can vary a lot and depends on what carbonate rock it originates from, it can have a pure milky appearance, foliated, can have various colorations, swirls and bands depending on impurities. From geotechnic standpoint granite is much tougher, harder rock (Europeans in fact couldn't work it for centuries in fine scale after antique steel tempering techniques were forgotten), it's a superb construction material. Marble is a pretty tough rock too and resistant to weathering, however it's soft enough that it can be carved with relative ease. Granite is also more common rock than marble. It's worthy noting many marble is not real marble in geological sense, but rather very cemented limestone (still offering very good, marble-like qualities). Romans liked a particular, rosy hard limestone they called "red marble' mined in Pannonia. Also it's not like granite wasn't used for such purposes - just think of Ancient Egypt who loved using granite for their monuments.

So there you have it. Granite is tough, more common (so less valuable), its grainy texture likely not so ideal for sculptures although I'm no artist to make proper comment on that. In contrast marble can be relatively easily worked and sculpted, and I suppose that peculiar silky, pure white marble is so prized because all the carved details stand out more from the whiteness (plus white can be painted easier), reflect light more. And not-pure white marble is beautiful too with all those colorful swirls and bands making it great for decoration.