He'd just been attacked by a magical source that disarmed his weapons at the wrists, was cast wordlessly and without wand motions, and did not leave an obvious bolt to block. This is probably a bad time to show off.

At a deeper level, Voldemort (and Mad-Eye Moody) favor a combat philosophy of dodging, under the quite reasonable realization that there are a large number of spells that can overwhelm shields or are unblockable.