The version of Ethereum after the hack became known as Ethereum Classic. The Ethereum foundation decided to go with a fork of the chain prior to the hack, and pretty much all the devs and the community followed. The value of Ethereum is entirely derived from what people are willing to pay for it, and community is a big part of that. The version of Ethereum which underwent the attack didn't cease to exist, and people can still use it; it's just called "Ethereum classic" now, whereas people who want to use the version of the chain that didn't suffer from the hack can use that version (generally understood to be "Ethereum".

The fact that there are far fewer users of Ethereum Classic (and the market cap is significantly lower) is a testament to how much people care about the community which chose to follow a different history of the Ethereum network.