No watercolors in the last few months, but...
There is a w/c among the top 15 in both Jan & Feb of this year, and a number of top 15 works in other media in most other months. The February judge is a watercolorist whose winners were all oil.
Dec 2015--a w/c got second place and another placed in the top 15.
October 2015--a watercolorist judged, and her winners also were all oil! Two of her top 15 were w/c.
Sept 2015--judged by a pastel artist; she chose three w/c's for her top 15 and gave another one second place.
In August 2015, Carol Marine chose a w/c for first place. Her top 15 included four w/c's, a colored pencil, a pastel, and an acrylic.
So based on the past year, which is a far back as I checked, I'm not sure your conclusion is accurate. Seems more likely it's a matter of oils far outnumbering other media in the works submitted.