Sure, Jennifer Lawrence was pretty good in Linings, but Chastain gave an insanely true performance. She invoked an amazing and powerful voice and didn't seem like a characatcher. She used emotions, and powerful ones. Lawrence came off a bit robotic at times. And yes, I know that was on purpose for her character, but I didn't buy it.
Jessica Chastain was not snubbed for best actress.
Firstly you have not stated in what awards ceremony you believe she was snubbed in, at the golden globes she won in her category which is by no means a snub. In the other category Jennifer Lawrence won meaning it could go either way at the baftas or at the Oscars.