Only because Blackberry's decline was so dramatic [1]. WP is growing very slowly. Plus, "3rd place" doesn't just mean "only one place behind". It means at least 15% market share behind, which is about 5 times what WP8 has now. Pretty big gap.

FF OS or even Ubuntu Touch could easily surpass it if they find the right partners, and especially if they focus more on the low-end, which FF OS seems to be doing right now.

[1] http://static3.businessinsider.com/image/5192a95969bedd70220...