Because as they mature, they grow bigger/better antlers each year..
In their growing/velvet stage, they're soft and sensitive, no good for fighting or even touching things with.. but once hard, good for fighting, poking holes in fences, and just showing off.
Obviously they need them hard in the rut, but they can't just switch back and forward between growing and hard, it's a one-way change. So they drop last years antlers and put up a new, better set. Also means if they break an antler off one year, they're not crippled for the fighting/mating game for life.
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