Etherum classic had three 51% attack this year and nobody seem to budge.
That is because nobody cares about altcoins at all. They all want to make profit from trading altcoins, in fact majority of people in crypto space don't even own any altcoins! They have custodial accounts that may hold some coins on their behalf (ie holding on exchanges) so that they can dump it at a moment's notice.
Altcoins are vulnerable to 51% attacks because they share mining algos with other altcoins (or with bitcoin), and the hashrate being utilized towards the altcoin is less than a majority of the hashrate for that algo.
In order to perform a 51% attack on bitcoin, an attacker would need to invest in mining equipment with mining capacity equal to >50% of the network hashrate. Once a 51% attacker executes his attack against bitcoin, the value of his mining equipment would decline substantially. This is not true for most altcoins.