I don't see how the substrate matters
The substrate has a large influence on the plasmon frequency of the gold which the nanoantennas are made of. The plasmon frequency in turn determines the size of the antenna. For many substrates they would have too small to either work or the be producible at all. Also, below a certain size you have to use fancy techniques such as electron beam lithography which is not really compatible with mass production.
Having said that, I'm certain that metamaterials will be a big think in the not-so-distant future.