Altcoin season is just a hope of repeating the past glory. That altcoin season was driven by blockchain and ICO hype as well as the growth of Bitcoin. Basically, investor thought that since Bitcoin is growing. Altcoin season means that alts grow faster than BTC during a bull run . because there is no way to explain why both Bitcoin and altcoins would be growing at the same time. And lastly, the usual scenario where Bitcoin would stop and altcoins will take over might not happen, because it is very clear that more attention is still on Bitcoin.
I think this time around it was more about defi, because there are so so so many defi projects and weirdly enough this time around it is not just a project that people invest and hope the price goes up neither, it is a project that "people have the ownership" and that is the deal here, and it doesn't end with the funding process. Back in 2017 period when ICO was a big deal, most people just bought those coins, funded them, got it for cheap and waited for the price to go up and then sold their coins and made a profit and that was the whole reason ICO existed, so that investors would make money.
Nowadays defi is much bigger, it is something that keeps on living, for example uniswap? That is still having a ton of people trading there, even after publishing and being released it is still used so much whereas most ICO coins were done in a year (most). So that is why people assume this could be different.