Its because they aren't 100% hardy and only survive best in warmer parts of the UK like Cornwall. I assume that theextreme temperatures parts of Scotland have had will of caused this problem this year.
Younger ones can be easily protected- Late autumn they should have all thier leaves bunched up together and loosely tied, then wrapperd in a layer of fleece to give it some protection. But I assume by saying mature, they are to big to do this.
Yours may come back, it all depends if the main growing point is damaged or not.
The plant may produce some side branched lower down the stem, but wont knw this till the weather warms and growth begins

like you mines have suffered this year because of the weight of the snow. I stay in Livingston and usually mine survive the Scottish winter fine but this year the volume of snow[around 18ins] has done the damage!!

Yes, the stem hasn't just been bent over by the weight of snow (which wouldn't happen), it is the long spell of intense cold that has destroyed the tissue in the stem. Shrubby monocotyledon's (which Cordylines are) don't have normal wood tissue like dicotyledons. Their stems are made of diffuse interwoven bundles of growth tissue and transport tubes for water and minerals separated by starchy material unlike the annually growing rings of dicotyledons that produce true wood. Once damaged by intense cold (ice crystals), it collapses having lost the turgidity that partly kept it upright.

The lower part of the stem often shoots freely from the base so don't remove it yet. Remember, the large rootball will want to replace it's former canopy quite quickly and any new basal shoots will grow much faster than replanting a new one. Leave it until at least the end of June.

sorry to disagree but i assure you that the weight of the snow was the cause of mine bending as it caused the same damage as if a heavy plank of wood had been put on top of it!