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.
In case of chain split, you will definitely gonna have double the bitcoin you have at present but not double the amount. All the coins from both the chains will most likely to have value lesser than what we have now. Chain split is the worst scenario possible. Most likely it wouldn't be happening. Everyone will most probably try to avoid split.