I kinda think it is based on what they are doing as in if they are doing just the music or including the theater part.

The just the actual opera music part seems to be more popular,the Minack Theater in the UK is literally cut out of the cliffs and performances are sold out.

Andrea Bocelli has a massive following with sell out crowds with a majority being in open air venues,as in literally closing of a large street and having a performance in Italy,Netherlands etc.

I think they are changing how opera is performed in order to draw more crowds and appeal to a broader audience,verses a venue where only the stuffy elite attend,settings like the derelict Michigan theater are more common place and more causal that attract a wider audience then the dress up to the nines for an evening at the opera.