Don't know why people buy anything but BTC. 
More like why would anyone buy a relatively new altcoin when the market is in the bear mode and every time BTC drops its price, that altcoin is more likely to drop further. Worse, that new one doesn't have a strong support base yet (new, remember?) so it is also more likely to get hit harder if fud/bad stuff happened to that altcoin which in this case, Optimism hacked.it's like a gamble and if i may suggest it is better to buy top coins which is not too risky,
but again everyone has their own decisions and most importantly they are responsible for their decisions