Because if there is a Hard Fork and a new Blockchain is created with Segwit activation, you will end up with FREE SEGWIT BTC. That's right, you will still have your Bitcoins PLUS the new Segwit coins on the new blockchain. If you have 10 BTC, you will have 10 BTC + 10 Segwit BTC. So if you were smart you would be acquiring as much Bitcoin as possible, especially while it is cheap, before the Hard fork.
That is my opinion.
there is no hard fork. and segwit is a soft fork.
and by the way in case a split happens and we end up with 2 coins i assure you the price will be a lot lower that it currently is. your total worth of 1 bitcoin will be less than $400 probably.
also it won't happen. there won't be a split. BIP148 either fails or miners activate SegWit. there won't be any other way.