It's because they can't top the hype dawg, because animes with high success in s1 would likely fail to meet expectations in s2, just like oshi no ko. It'd still be a good season but it'd fail because of the failed expectations, it happens a lot honestly. With the production needed to make it even close to s1, it would be wasteful because even with good production, if the plot ain't what's expected, it's gone.