Because she wouldn't have had any time to drop the knife in the pool in that case! We know it had to have been dropped between 7:30 and 10, but Lucy says she was with Clover that entire time - and if she's lying, then she wouldn't have accidentally turned on the stove in the first place! [UGH PROTECT GLITTERSIS] Unless you think the killer didn't use the knife at all and chucked it in the pool way earlier to throw us off, but then what did they slash the pipes with?

It's...possible I guess, but still risky. Plus she wouldn't have had time to drop the scarf into the dryer unless she did it earlier before finding Cheshire. And we still run into the same problem with her as we do with Clover when it comes to the knife in the pool.