It was as visible as daylight since the Capello era that Chiellini isn't a good LB, especially when going forward. In 2005, we had the likes of Cannavaro and Thuram in the CB position, so obviously he could only get playing time playing LB. Deschamps noticed his strong defending ability and we could afford to play him there since there was no competition whatsoever, and it worked out well for both parties.
I do agree that currently he's the best option we've got for the LB position. We should get a proper LB and move him back in the centre though.