I wouldn't be surprised if it's because she won so recently - it has only been 5 years (or 6?), so they may feel that they shouldn't recognise her again just yet - it took them 30 years to give Meryl Streep her 3rd!
I don't feel that is an excuse really
For example, Jodie Foster and Hilary Swank won both their oscars in between a few years of each other. I think there was two years between The Accused and SIlence.
Whether they have won recently or not, it isn't an excuse for someone to be nominated. It took them 30 years to give Meryl another oscar because nearly every damn film she gives the performance of her life so they were spoilt for choice.