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.
It is true that the projects in 2018 are very few compared to the current number of projects. But most of them are also dead and only a few projects actually survive and develop to this day. Apart from ETH, BNB, LTC, and some other projects, the rest I don't see anyone mentioning them in the market.
Bitcoin is still the most reliable cryptocurrency, so it is understandable that it still exists, moreover, it has the largest market cap if it crashes, then the market crashes.