I don't watch a lot of adverts, but I would imagine that car makers will generally only advertise their newer/upgraded cars or those that are in high production numbers. They will have a limited ad budget, so will need to target it on those cars that will bring them most profit.
VX are being quite open about pushing to become the No1 car brand in the UK by 2020, so they are always going to be pushing the mainstream products- Corsa, Astra and with the Adam and Mokka, they have new trendy products that will appeal to the youngest end of the market. If they can get the youngsters on board, they garner themselves a trendy image and hopefully make lifetime customers out of the young ones (as they did with me all those years ago!- i am now awizened old 38 year old!!). Niche products like the Antara are not going to be pushed heavily as the market is quite small and a reasonable number of orders trickle through without any advertising spend. I am sure when the new model arrives there will bea bit ofan initial splash and the marketing will tail off as it has done with this version.