Since there weren't as many cryptocurrencies back in 2018 as there are now, most of them survived the crash because there weren't this many shitcoins and useless projects at that time. Most of the coins that have been on the top along with Bitcoin survived though falling badly in price since Bitcoin had a very severe crash.
About why Bitcoin survives the crashes, it's mostly because there is a lot of money invested in it and as soon as it falls lower in value, there is higher buying pressure which basically backs it up and it manages to recover slowly from the crash every time.