It doesn't necessarily need segwit, rather it needs a fix for transaction malleability, which segwit includes. LN uses advanced smart contracts which don't work well when the transactions can be malleated. There are other proposed fixes for malleability other than segwit, however segwit is the most tested solution, though segwit is not the absolute cleanest solution, in particular the fee discount makes spam a bit easier, but in exchange gives a softfork blocksize increase.