I'm not entirely certain why this is. My guess is that the type of people who like to recreate historical costumes are less likely to wear them as cosplays and more likely to wear them to historical balls, which are less likely to find their way onto the internet. Maybe? There are plenty of holes you could poke into that theory.
Another, perhaps more likely guess is that Edwardian era costumes work best on a very specific type of figure - a boyish one. If you're like me and have a curvy figure, then as much as you might adore the fashions, you know it'll be a tough sell to pull one off and feel like you're looking like the character. The costumes are rather too elaborate to put a lot of work into if one isn't going to feel confident wearing one.
It could also have something to do with the overall popularity of the show making cosplayers less interested. "Downton Abbey" has become so mainstream, it's hardly "hipster" to do a Crawley cosplay. And with the loss of two fan favorite characters in season 3, it shook up the core fandom who had been with the show from the beginning and were the demographic most likely to make costumes.
Another factor could be a lack of easily accessible sewing patterns. Simplicity and McCalls are late in jumping onto the bandwagon here. Whereas people have been making Superhero costumes for years, and most "Once Upon a Time" costumes can be easily adapted from existing patterns, "Downton Abbey" is a specific style with little help in the mainstream pattern world (although more are coming soon!) So there is some hope that we might see an upsurge in Crawleys and Bates soon enough.
In the meantime, take a look at what has been done, and done pretty well too!