Why are higher oxides of non-metals more acidic?
It is expected oxides with more polar $\ce{M-O}$ bonds are stronger Lewis bases than those with less polar bonds.

Furthermore, for the same element, the polarization gets weaker/Lewis alkality weaker when more $\ce{O}$ atoms are attached.

When hydrated to acids, another phenomena plays a role in acidity: a charge delocalization.

It is well observable that acids like $\ce{HNO3}$, $\ce{HClO3}$, $\ce{HClO4}$, $\ce{H2SO4}$, $\ce{HBF4}$ are strong, as the delocalization  has a good sandbox to play in.

It also plays role in solubility of salts in organic polar solvents, where there is a large  organic cation with one of the above small anions.